Can pathogens enter the bloodstream if the epidermis has been scraped off?

Can pathogens enter the bloodstream if the epidermis has been scraped off?

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Assume that the epidermis has been scraped off by a fall (not just scratched). Also assume that the dermis is completely untouched. Does this allow pathogens to enter the body more easily, or does the dermis still provide adequate protection?

Our epidermis is the first line defence against natural infections; it is also a part of the innate immune system. This is due to

  1. it contains a layer of dead cells that separates the living cells of the deeper layers of epidermis and dermis from the environment.

  2. This part of the epidermis is avascular and so pathogens can not easily enter into the bloodstream through it.

  3. It contains natural bacterial flora (like in our gut) that kills a large number of pathogenic microorganisms with their toxins.

  4. secretion of sebaceous gland contains some antimicrobial substances.

There are lots of other protective features of epidermis.

Dermis is richly supplied by blood vessels. So even if all the layers of dermis remain intact its high vascular nature will act as an open entrance of microorganisms. By definition abrasion involves only the epidermis or some parts of the dermis but still we need to take tetanus vaccine after getting an injury contaminated with soil. In diseases like Stevens-Johnson Syndrome (where epidermis separates from dermis) or in burn patients the skin is covered with antibiotics and dressings. The patient is also given high dose of antibiotics to fight those bacteria that still entered into the bloodstream.

This answer is a reply to the question asked in the comment of my answer given on 13th July 2016. The comment was, " Nice answer, I gather that bacteria would enter more easily, what about viruses?"

In my answer on 13th July 2016, I used the term "microorganisms" - this includes both bacteria and viruses. So the answer provided there can also be applied to viruses.

To specifically discuss on the susceptibility of the host towards viral infections after damage to the epidermis (dermis is intact) the following may prove to be helpful.

Viruses has evolutionarily developed specific surface molecules that either act as a ligand or a receptor being capable of interacting with the host surface-molecules. Example - Influenza virus contains Haemagglutinin that bind to Sialic acid containing oligosaccharides found in almost every cell. Common entry route of a virus is usually determined by the surface molecules it possesses. Most common entry routes are respiratory system, gastrointestinal system, urogenital system and inoculation by bite of insects(via insect probosis). Viral infections after skin-damage are rather uncommon.

Intact skin is an effective barrier against viral infections. Dead cell layer of epidermis prevent viral growth. Viruses are non living particles outside leaving cells; so when they come in contact with the dead cellular layer of epidermis they behave in the same way as they were in outside environment. The requirement of blood or lymph supply for replication and spread of viruses meets when epidermis is removed. But in that case there occurs an inflammatory reaction causing accumulation of immune cells and antibodies. These antibodies neutralize the viral surface molecules, thus preventing viral attachment and entry into cells ( even in presence of living cells). Immune cells secret chemicals that disintegrate viral particles.

Most common mechanism of entry into the skin, for a viral pathogens are, 1. direct inoculation 2. systemic infection and 3.local spread from an internal focus

  1. Direct inoculation is our topic. It uses two mechanisms - (i) replication in endothelial cells and then spread into the bloodstream. Entry into endothelial cells are different for enveloped and non-enveloped viruses; enveloped virus fuses with the cell membrane, while non-enveloped viruses enter by endocytosis or receptor-mediated processes. Examples include warts, milker's nodules, primary herpes simplex etc. (ii) Vectors may introduce the pathogen directly into the bloodstream

  2. Systemic infections- Skin is infected during high viremia. Dermis is affected first followed by epidermis. Example- chicken pox.

  3. Local spread from an internal focus- Example, recurrent herpes simplex virul infection from dorsal nerve roots to specific skin dermatomes.

For further infomations you may read 1. Fitzpatrik's dermatology in general medicine, 8th edition, 2. Kuby Immunology, 7th edition.

The dermis provides some protection, and if it is removed the danger of deep tissue infection becomes extremely high. However, the epidermis provides an additional and important layer of antimicrobial protection. That is why first and second degree burns, opened blisters, and scrapes should be kept clean and isolated from the environment with bandages, and why antimicrobial ointments are often prescribed in such cases. The epidermis is an important waterproof barrier to environmental hazards, such as germs and dehydration, and its dryness is a natural inhibitor of microbial growth. Also:

Understanding the Epidermis

Heather L. Brannon, MD, is a family practice physician in Mauldin, South Carolina. She has been in practice for over 20 years.

Casey Gallagher, MD, is board-certified in dermatology. He is a clinical professor at the University of Colorado in Denver, and co-founder and practicing dermatologist at the Boulder Valley Center for Dermatology in Colorado. His research has been published in the New England Journal of Medicine.

The epidermis is the outermost layer of the skin. The thickness of the epidermis varies depending on where on the body it is located. It is at its thinnest on the eyelids,   measuring just half a millimeter, and at its thickest on the palms and soles at 1.5 millimeters.

Other Ways Tetanus Gets Into Your Body

Tetanus bacteria can also infect the body through breaks in the skin caused by:

  • Clean superficial wounds (when only the topmost layer of skin is scraped off)
  • Surgical procedures
  • Insect bites
  • Dental infections
  • Compound fractures (a break in the bone where it is exposed)
  • Chronic sores and infections
  • Intravenous (IV) drug use
  • Intramuscular injections (shots given in a muscle)


The maintenance of skin integrity is crucial to ensure the physiological barrier against exogenous compounds, microorganisms and dehydration but also to fulfill social and aesthetic purposes. Besides the development of new actives intended to enter a formulation, innovative technologies based on physical principles have been proposed in the last years. Among them, Cold Atmospheric Plasma (CAP) technology, which already showed interesting results in dermatology, is currently being studied for its potential in skin treatments and cares. CAP bio-medical studies gather several different expertise ranging from physics to biology through chemistry and biochemistry, making this topic hard to pin. In this review we provide a broad survey of the interactions between CAP and skin. In the first section, we tried to give some fundamentals on skin structure and physiology, related to its essential functions, together with the main bases on cold plasma and its physicochemical properties. In the following parts we dissected and analyzed each CAP parameter to highlight the already known and the possible effects they can play on skin. This overview aims to get an idea of the potential of cold atmospheric plasma technology in skin biology for the future developments of dermo-cosmetic treatments, for example in aging prevention.

Garden injuries

Cuts and scrapes are the most likely wounds gardeners incur (hopefully, not on that green thumb of yours). In many cases, these could have been prevented by simply using hand protection. Start off your gardening efforts by obtaining a good pair of work gloves most injuries will occur on your hands.

You can expect to be at risk for the following mishaps in the garden:

Minor cuts (scratches): These tears in the skin only penetrate the “epidermis” (superficial skin layer) and become infected on an infrequent basis in a healthy person.

Abrasions (scrapes): A portion of the epidermis has been scraped off, exposing the dermis. You probably have experienced plenty of these as a child.

Contusions (bruises): These result from blunt trauma and do not penetrate the skin at all. However, there is bleeding into the skin from blood vessels that have been disrupted by the impact.

All of the above minor injuries can be easily treated. Wash the wound anywhere that the epidermis has been violated. The use of an antiseptic such as Betadine (Povidone-iodine solution), honey, or triple antibiotic ointment, such as Neosporin or Bactroban, will be helpful to prevent infection. Ibuprofen and acetaminophen are useful over-the-counter drugs to treat minor pain.

Minor bleeding can be stopped with a wet styptic pencil, an item normally used for shaving cuts. The wound, if it broke the skin, should have a protective adhesive bandage (such as a Band-Aid) to prevent infection.

Applying pressure and ice wherever a bruise seems to be spreading will stop it from getting bigger. Bruises will change color over time from blackish-blue to brown to yellow. Bruises may be gravity-dependent and may descend slightly as time goes on.

The Liquid Skin bandage is an excellent way to cover a minor injury with some advantages over a regular bandage. Apply it once to the cut or scrape it dries within a minute or so and seals the wound. It also stops minor bleeding and won’t peel off during baths. There are various brands (Band-Aid Liquid Bandage, New Skin, Curad, 3M Nexcare No-Sting liquid bandage) and many come as a convenient spray. These injuries will heal over the next 7-10 days, dependent on the amount of skin area affected.

If you have one of the minor injuries mentioned above, why not consider natural remedies? Here’s an alternative process to deal with these issues:

1) Stop minor bleeding with herbal blood clotting agents and compress the area with gauze. Substances that clot blood are called “hemostatic” agents. These include:

  • Essential oils — geranium, helichrysum, lavender, cypress, myrrh, or hyssop.
  • Medicinal herbs — yarrow tincture, or directly apply cayenne pepper powder or cinnamon powder.

2) After minor bleeding is stopped, the wound should be cleaned with an herbal antiseptic. Mix a few drops of oil with sterile water and wash out the wound thoroughly. Essential oils with antiseptic properties include:

Apply herbal antiseptic to the wound in a 50/50 mix with carrier oils such as olive or coconut oil. Other natural antiseptics include garlic, raw unprocessed honey, echinacea, witch hazel, and St. John’s wort.

3) If needed, use natural pain relievers such as:

  • Geranium oil
  • Helichrysum oil
  • Ginger oil
  • Rosemary oil
  • Oregano oil

Apply 2-4 drops of a 50/50 dilution around the wound’s edges.

4) Dress the wound using clean gauze. Do not wrap too tightly.

5) Twice daily, change the dressing, reapply antiseptic, and observe for infection until healed.

An infected wound will appear red, swollen, and warm to the touch. In these cases, the use of antibiotics such as Amoxicillin 500 mg orally for 7-10 days is curative.

Second degree burn (Photo: CJR80)


Another common medical problem a gardener will be at risk for is sunburn. The severity of the burn injury depends on the percentage of the total body surface that is burned, and on the degree (depth) of the burn injury.

First degree burns are very common, such as simple sunburn. The injury will appear red, warm, and dry, and will be painful to the touch. These burns frequently affect large areas of the torso immersion in a cool bath is a good idea or, at least, running cool water over the injury.

Placing a cool moist cloth or Spenco 2nd Skin on the area will give some relief, as will common anti-inflammatory medicines such as Ibuprofen. Aloe vera or zinc oxide cream is also an effective treatment.

Usually, the discomfort improves after 24 hours or so, as only the superficial skin layer, the epidermis, is affected. Avoid tight clothing and try to wear light fabrics, such as cotton.

Second degree burns are deeper, going partially through the skin, and will be moist and have blisters with reddened bases. The area will have a tendency to weep clear or whitish fluid. The area will appear slightly swollen, so remove rings and bracelets.

To treat second degree burns:

  • Run cool water over the injury for 10-15 minutes (avoid ice).
  • Apply moist skin dressings such as Spenco 2nd Skin.
  • Give oral pain relief such as Ibuprofen.
  • Apply anesthetic ointment such as Benzocaine.
  • Use silver sulfadiazine (Silvadene) creams to help prevent infection.
  • Consider antibiotic ointment if slow to heal.
  • Lance only large blisters.
  • Avoid removing burned skin.

I had a significant second degree burn as a child (they called it “sun poisoning” back then) and my little brother thought it was a good idea to peel off some skin. He ended up with a 10-inch x 2-inch strip of skin in his hands. Do not peel off skin from a second degree burn.

There is a deeper degree of burn known as “third degree.” This is a major injury that is highly unlikely from simple exposure to the sun.

Again, an ounce of prevention is worth a pound of cure. To avoid getting burned:

  • Do not sunbathe (a tan is not healthy).
  • Avoid peak sun hours (11 a.m. to 4 p.m.).
  • Wear long pants and sleeves, hats, and sunglasses.
  • Spend some time in the shade.

If you cannot avoid extended exposure to sunlight, be certain to apply a sunblock. It should be applied prior to going outside and frequently throughout the day. Even water resistant/proof sunscreens should be reapplied every 1 to 2 hours. Most people fail to put enough on their skin be generous in your application.

A sunblock and a sunscreen are not the same thing. Sunblocks contain tiny particles that “block” and reflect UV light. A sunscreen contains substances that absorb UV light, thus preventing it from penetrating the skin. Many commercial products contain both.

The SPF (Sun Protection Factor) rating system was developed in 1962 to measure the capacity of a product to block UV radiation. It measures the length of time your skin will be protected from burning.

A SPF (sun protection factor) of at least 15 is recommended. It takes about 20 minutes without sunscreen for your skin to turn red. A product that is SPF 15 should delay burning by a factor of 15, or about 5 hours or so. Higher SPF ratings give more protection, and might be beneficial to those with fair skin.

Those experiencing an anaphylactic reaction will require treatment with epinephrine. An EpiPen has a pre-measured dose cartridge for quick delivery.

Insect bites

There so many insects, both beneficial and harmful, in the garden that you can expect to regularly get bitten by them. Insect bites usually cause pain with local redness, itching, and swelling but are rarely life-threatening.

For most victims, the offender will be a bee, wasp, or hornet. A bee will leave its stinger in the victim, but wasps take their stingers with them and can sting again. Even though you won’t get stung again by the same bee, they send out a scent that informs nearby bees that an attack is underway. As such, you should leave the area whether the culprit was a bee, wasp, or hornet.

The best way to reduce any reaction to bee venom is to remove the bee stinger as quickly as possible. Pull it out with tweezers or, if possible, your fingers. The longer bee stingers are allowed to remain in the body, the higher chance there is for a severe reaction.

Most bee and wasp stings heal with little or no treatment. For those who experience only local reactions, the following actions will be sufficient:

1. Clean the area thoroughly.

2. Remove the stinger if visible.

3. Place cold packs and anesthetic ointments to relieve discomfort and local swelling.

4. Control itching and redness with oral antihistamines such as Benadryl or Claritin.

5. Give acetaminophen or ibuprofen to reduce discomfort.

6. Apply antibiotic ointments to prevent infection.

Topical essential oils may be applied (after removing the stinger) with beneficial effect. Use Lavandin, helichrysum, tea tree, or peppermint oil, applying 1 or 2 drops to the affected area, 3 times a day. A baking soda paste (baking soda mixed with a small amount of water) may be useful when applied to a sting wound.

Although most of these injuries are relatively minor, there are quite a few people who are allergic to the toxins in the stings. Some are so allergic that they will have what is called an “anaphylactic reaction.” Instead of just local symptoms like rashes and itching, they will experience dizziness, difficulty breathing, and/or faintness. Severe swelling is seen in some, which can be life-threatening if it closes the person’s airways.

Those experiencing an anaphylactic reaction will require treatment with epinephrine as well as antihistamines. People who are aware that they are highly allergic to stings should carry antihistamines and epinephrine on their person whenever they go outside.

Epinephrine is available in a pre-measured dose cartridge known as the “EpiPen” (there is a pediatric version, as well). The EpiPen is a prescription medication, but few doctors would begrudge a request for one. Make sure to make them aware that you will be outside and may be exposed to possible causes of anaphylaxis. As a matter of fact, it may be wise to have several EpiPens in your possession if you have allergies.

Other medical issues for the gardener include spider bites, snake bites, mosquito-borne illness, and pathogens (disease-causing organisms) that reside in the soil. Next issue, we’ll explore these subjects.

Construction principles

The skin of all fish species, like that of any other vertebrate, consists of two basic layers: an outer, the epidermis, and an inner, the dermis (Figs 1 and 2). The entire outer surface of a fish, including the body and fins, is completely covered by epithelium. Beneath its protective mucus, which is also very important against hydrodynamic drag, cell-cell-junctions of the stratified squamous superficial epithelial cells, also called filament cells or Malpighian cells, provide the only epithelial coherence. Investigating the functions of tight junctions in fish skin could be directly relevant for investigative dermatologists, because the role of tight junctions, which are expressed not only by keratinocytes but also by epidermal Langerhans cells, is of outstanding recent interest in human epidermal biology ( 2, 150, 151 ). In fish, this boundary is interrupted by mucus-secreting cells. All of these cells are part of the stratum superficiale, the outermost part of three epidermal strata, followed by the stratum spinosum medial with differentiated cells and proximal the stratum basale with basal cells and the basement membrane. The thickness of these epidermal structures is often bound to ecological factors, to seasonal variances or to variances between males and females. Undifferentiated epidermal progenitor cells emerging from the basal layer are induced to proliferate and differentiate in the stratum spinosum when needed and subsequently recruited to the outermost epidermal layer. Depending upon the fish species, fish age, location on the body, epidermis thickness and number of epidermal layers, various specialized cells, including goblet cells, sensory cells, alarm cells and chloride cells may be present in the epidermis ( 61 ).

Fish epidermis is separated from the underlying dermis by a layer of filamentous proteins, which form the basement membrane.

The adjacent dermis is composed of the stratum laxum and the stratum compactum and, in striking contrast to mammalian skin, is separated from the hypodermal adipogenic tissue by yet another endothelial layer, called the dermal endothelium ( 64 ). The thin stratum laxum of fish dermis consists of a loosely arranged connective tissue, complemented with blood vessels and nerve fibres. Dermal cells are mostly fibroblasts, interspersed with different chromatophores. Scale-building cells, the scleroblasts of fish skin, are arranged in the scale pockets. The most frequent scale type in teleosts is the elasmoid scale (Fig. 3), which consists of a plate of collagenous tissue, with superficial mineralization, surrounded by scleroblasts and fibroblasts ( 61 ). On the lower side, the scale pocket is lined by modified fibrocytes with desmosomes and caveolae. Bundles of collagen fibres anchor the scale in its pocket. The posterior edge of the scale is more or less covered by the epidermis, depending on the species and their biology.

Regions of dermo-epidermal interactions in the fish skin may be present like it is supposed for reptilians ( 148 ). Here, morphoregulatory molecules are exchanged and may then have significant influence on the structural composition of the epidermis and dermis. In mammals, small dermo-epidermal connected regions migrate into the dermis and form dermal papillae and then hairs ( 148 ).

Stratum Basale

The bottom layer of the epidermis is called the stratum basale. This layer contains one row of column-shaped keratinocytes called basal cells. These cells are constantly dividing and pushing already-formed cells towards the skin's surface. As basal cells move into the upper layers, they will also flatten, die, and be shed to make room for newer cells.

Melanocytes, the cells that produce melanin (the pigment which provides your skin its color), are also found in this layer.

Lesson Explainer: The Nonspecific Immune Response Biology

In this explainer, we will learn how to describe how the body prevents pathogen entry and explain ways in which the nonspecific immune system responds to a pathogen invasion.

They say the best offense is a good defense. Luckily, the human body has several defensive systems that prevent our bodies from having to fight infections by simply keeping most pathogens out! The nonspecific immune response is the body’s method of preventing the entry of pathogens as well as rapidly responding to the first signs of potential infection.

A pathogen is an organism or agent that can infect a host and cause disease. Common pathogens include microorganisms like bacteria or fungi and nonliving entities like viruses, prions, and viroids. An antigen is a substance that initiates an immune response. Antigens and pathogens are related. Pathogens may have antigens on their surface or secrete antigens such as toxins that are dangerous to cells. The relationship between pathogens and antigens is shown in Figure 1.

The immune system can act as a barrier to pathogens, and it can do this in physical or chemical ways. These physical barriers involve the epithelium of the skin because these cells form tight junctions between each other to physically prevent pathogens from entering the body. Chemical barriers include the actions of the immune cells, the chemicals they produce, and other substances within the body that act to slow, damage, or destroy the pathogens that may have entered the body.

There are two main types of immune responses: nonspecific and specific. Specific immunity is also called adaptive immunity or acquired immunity. This is the part of the immune system that adapts to respond to a specific threat in a specific way. We call this type of response antigen specific. Adaptive, specific, immunity develops with your body over time.

Nonspecific immunity is also called innate immunity. It is the part of the immune system that reacts to every threat in the same way. Innate immunity is not antigen specific. Innate, nonspecific, immunity is immunity you are born with. A flow chart outlining the differences between innate immunity and adaptive immunity is shown in Figure 2.

Key Term: Nonspecific Immunity (Innate Immunity)

Nonspecific immunity is immunity you are born with that protects you against all pathogens in the same way.

Nonspecific, or innate, immunity begins with the barrier defenses, which are barriers that protect the internal environment from foreign particles and potential pathogens. These defenses can also be described as the “first line of defense.”

Every point of entry to our bodies (for example, our skin, eyes, ears, nose, and digestive system) has a specialized defense of its own.

The skin defines the boundaries of the human body. It covers and protects all of the external surfaces. Our skin also houses immune cells, such as mast cells, that rapidly respond to infection if the barrier is broken by an injury, such as a cut or a scrape.

Key Term: Mast Cell

A mast cell is an immune cell found in the skin that releases histamine in response to injury.

The surface of the skin is home to a community of bacteria that are harmless and sometimes beneficial. However, these same bacteria can pose a threat to our internal environment. This is one reason why the skin must function as a robust defense against invasion by potential pathogens.

While the skin protects the external surfaces, mucous membranes serve that same purpose when it comes to body cavities and internal surfaces. Mucous membranes are composed of one or more layers of epithelial cells, and they generally secrete mucus of some sort. The types of cells and types of mucus secreted vary by location. Mucous membranes are found in the eyes, ears, nose, mouth, vagina, urethra, and anus. The mucous membranes function to prevent dirt and pathogens from entering the body. A diagram of a mucosal epithelium is shown in Figure 3.

The mucous membranes of the ear are found in the middle and inner ear, the parts deep within our head. The outer ear produces a material called cerumen, also known as earwax. Cerumen is an additional barrier method of protection that prevents foreign materials from entering the ear. It has also been shown to have some antimicrobial and antifungal properties.

The mucous membranes of the eyes are found on the inner surfaces of the eyelids. The eyes are also protected and lubricated by tears. Tears are secretions of glands called lacrimal glands, or tear glands, found in the tissue surrounding the eye. The lacrimal glands are constantly secreting fluid that keeps the surface of the eye moist. This fluid also contains enzymes that can destroy certain bacteria. One of these enzymes is called lysozyme, and it has the ability to break open, or “lyse,” certain bacteria to kill them.

The lacrimal glands make more fluid when the surface of the eye is stimulated by contact with a foreign object, like dust, which helps to flush these objects from the eye. Dust and dirt can carry pathogens as well as cause physical damage to the delicate surface of the eye. The lacrimal glands also make the tears humans cry when we experience extreme emotions like joy or sadness.

Nearly the entire surface of the respiratory tract is lined with mucous membranes, from the nose and sinuses to the inside of the lungs. Mucus moistens the surfaces of the lungs, which allows dissolved gases like oxygen and carbon dioxide to pass through. Mucous membranes in the lungs also serve the purpose of trapping foreign particles and potential pathogens.

Key Term: Mucous Membrane

A mucous membrane is a specialized kind of epithelial tissue with cells that secrete mucus and can have ciliated cells that move the mucus from place to place.

Example 1: Recalling the Organs That Possess Goblet Cells

Goblet cells, like the one shown in the diagram, are mucus-secreting cells. They are present in mucous membranes throughout the body and the mucus they release traps pathogens and other foreign particles.

Which organ does not contain goblet cells?


Goblet cells are cells specialized to secrete a fluid called mucus. These mucus-secreting cells are found in mucous membranes. Mucous membranes possess a specialized type of epithelium, or lining tissue, that has some cells that are specialized to make mucus. Some mucous membranes also contain ciliated cells that are specialized to move the mucus from place to place.

In the immune system, mucous membranes function as an aspect of innate, or nonspecific, immunity. Innate immunity is immunity that responds to every pathogen in the same way. Another way to say this is that innate immunity is not antigen specific.

Innate immunity involves barrier immunity or physical and physiological factors that prevent pathogens from entering the body. If a pathogen makes it past the barriers, or first line of defense, the second line of innate immune defense is activated. This involves the action of some cells and chemical components that fight the pathogen to halt or slow the spread of infection.

Mucous membranes are barrier methods of immune defense. They are present as linings to most body cavities and points of entry into the internal environment of the body. The mucous membranes function to prevent dirt and pathogens from entering the body. Mucous membranes are found in the eyes, ears, nose, mouth, vagina, urethra, and anus.

This means that the organ that does not contain goblet cells is the skin.

The mucus in our airways is constantly moved by special cells called ciliated cells. These cells have hairlike structures that sweep the mucus in one direction, toward the throat. Anything trapped in the mucus of the airways is eventually swallowed and passed to the stomach. Excess mucus can cause coughing, which will more forcefully expel mucus and trapped foreign particles from the body through the mouth.

Coughing is an example of an expulsive reflex. Another example is sneezing. Expulsive reflexes are reflexes that forcefully expel foreign substances from the body. Coughing expels material from the airways leading to the lungs. Sneezing does the same for the nasal passages. Some pathogens, like respiratory viruses such as COVID-19, use these expulsive reflexes to their advantage to help them spread to new hosts.

Like the respiratory system, the digestive system is also nearly completely lined with mucous membranes. These membranes serve as barriers that protect the internal environment from foreign particles and potential pathogens. In addition to the protection the mucous membranes provide, different parts of the digestive system have additional defenses of their own.

For example, the saliva produced in the salivary glands of the mouth has enzymes in it that can destroy some types of bacteria.

The gastric juices of the stomach (

) have an extremely low pH, which means that they are very acidic. This acid serves to destroy most pathogens that enter the stomach either on our food or in the mucus that is transferred from the respiratory tract.

At every potential point of entry to the body, there are barriers that protect the internal environment from foreign particles and potential pathogens. This is what we sometimes call the first line of defense. However, there are some instances in which pathogens still manage to enter the body. This would activate the “second line of defense.”

Example 2: Identifying Physical Barriers to Pathogens

Which statement is an example of a physical barrier to pathogen entry?

  1. Complement proteins in blood
  2. Keratinized layers of epidermis
  3. Hydrochloric acid in the stomach
  4. Oils and salt in sweat
  5. Lysozyme in tears and saliva


The innate immune system, or nonspecific immunity, describes the part of the immune system that is not antigen specific. These are barriers and physiological responses that respond to every pathogen in the same way.

The barriers that are a part of the innate immune system are what we call the first line of defense against pathogens. They function to prevent pathogens from entering the body where they can cause damage and develop into illness and disease.

Some barriers are physical and keep foreign particles and pathogens out. Some are chemical like chemicals, enzymes, and other substances that slow, damage, or destroy pathogens that may enter or have entered the body.

Out of the answer choices presented, complement proteins in blood, hydrochloric acid in the stomach, oils and salt in sweat, and lysozyme in tears and saliva are all examples of chemicals and other substances. Many of these function as barriers to pathogens, but they are not purely physical in nature.

The keratinized layers of epidermis describe the outer layers of the skin. The skin is the physical barrier that protects all of the external surfaces of the body. The outer layers of cells are flat, irregular, and overlapping. They form a waterproof layer that is very effective at preventing the entry of pathogens.

This means that the example of a physical barrier is the keratinized layers of epidermis.

Imagine that you accidentally scrape your arm against something sharp and cut your skin. At first, you will experience bleeding and pain. The blood flushes pathogens out to prevent them from entering the blood vessels. The bleeding stops when a clot forms, which is a mass of blood cells that plug the wound. The blood clot contains many immune cells. This clot eventually forms a scab, which further protects the internal environment from invasion.

In addition to this response, you may almost immediately experience swelling, redness, and pain at the site of the wound that last for a number of hours or days after your initial injury. The heat, redness, and pain are all symptoms of an innate immune response called the inflammatory response, or inflammation.

Key Term: Inflammation

Inflammation is a reaction to injury or infection in which part of the body becomes reddened, swollen, painful, and hot.

We mentioned that the skin is home to immune cells, including mast cells. When these cells detect damage or injury to cells in the skin, they release histamine and cytokines. Cytokines are chemicals that facilitate communication between cells during an immune response. The cytokines and histamines initiate the inflammatory response. A diagram illustrating the stages of an inflammatory response after injury to the skin is shown in Figure 4.

Histamine causes the capillaries and blood vessels in the skin to increase in diameter, allowing more blood to flow to the area around the wound. This is called vasodilation. The vasodilation is the source of the heat and redness. The word inflammation literally means “to inflame, or become hot.”

Histamine also makes the capillaries more permeable, allowing more fluid to leave the bloodstream and enter the tissues of the skin. This is the source of the swelling.

Key Term: Histamine

A histamine is a compound that is released by cells in response to injury and in allergic and inflammatory reactions. Histamine causes vasodilation.

Key Term: Vasodilation

Vasodilation is when blood vessels increase in diameter, which increases blood flow to an area.

Example 3: Recalling the Effect of Histamine on Blood Vessels

How does histamine affect blood vessels near an injured area?

  1. It constricts blood vessels but does not affect capillary permeability.
  2. It constricts blood vessels and increases capillary permeability.
  3. It dilates blood vessels and decreases capillary permeability.
  4. It dilates blood vessels but does not affect capillary permeability.
  5. It dilates blood vessels and increases capillary permeability.


There are two main types of immune responses: nonspecific and specific. Specific immunity is also called adaptive immunity or acquired immunity. This is the part of the immune system that adapts to respond to a specific threat in a specific way. Adaptive, specific, immunity develops with your body over time.

Nonspecific immunity is also called innate immunity. It is the part of the immune system that reacts to every threat in the same way. Innate, nonspecific, immunity is immunity you are born with.

Innate immunity begins with barrier defenses. These are barriers that protect the internal environment from foreign particles and potential pathogens. Skin is one of the body’s barriers. When the skin is damaged, the barrier is broken, and pathogens, like bacteria, can enter the body.

Histamine is released by mast cells and some other cells in response to injury to the skin or another part of the body. When our skin is damaged by a cut or a scrape, damaged cells send signals that trigger mast cells to release a chemical called histamine. The histamine sets into motion a series of events we collectively call an inflammatory response.

Inflammation literally means “to inflame.” It refers to the signs of inflammation, which are redness, heat, swelling, and pain. The histamine causes blood vessels near the site of the injury to become larger in diameter, which is called vasodilation. This allows more blood to flow through. This causes heat and redness. The histamine also triggers the blood vessels to become more permeable. This means that materials can pass into and out of the blood vessels more easily. Fluid from the blood is transferred to the tissue surrounding the injury, which leads to swelling. Additionally, cells called phagocytes migrate from the bloodstream to the injured area where they engulf any pathogens that may have entered the body.

Using this information, we can conclude that histamine affects blood vessels because it dilates blood vessels and increases capillary permeability.

The early stages of inflammation attract phagocytes: immune cells that can engulf pathogens using a process called phagocytosis. Phagocytosis forms a vesicle around the pathogen that is moved to the interior of the cell. Then, the pathogen can be destroyed or broken apart to allow the antigens to be displayed on the cell surface.

Phagocytosis proceeds according to the following stages:

  1. The pathogen is attached to the cell membrane.
  2. The pathogen is engulfed by the cell.
  3. The phagosome is formed.
  4. The phagosome and lysosome fuse to form phagolysosome.
  5. Lysosome enzymes neutralize and break apart the pathogen.
  6. Digested pathogen products are either released from the cell or presented on the cell surface as antigens.

The stages of phagocytosis are illustrated in Figure 5.

Phagocytic cells include macrophages, neutrophils, and other similar cells. These cells are able to easily migrate to the site of infection due to the increased blood flow and increased permeability of the capillaries.

Key Term: Phagocytosis

Phagocytosis is a form of endocytosis in which cells engulf and internalize solid matter such as a pathogen.

Example 4: Identifying Components of the Inflammatory Response to Skin Injury

The figure shows the cellular response to tissue injury after the skin has been broken by a nail.

What process is illustrated by cell X?

  1. Histamine release
  2. Phagocytosis
  3. Chemotaxis
  4. Edema (swelling)
  5. Clot formation


When our skin is damaged by a cut or a scrape, damaged cells send signals that set into motion a series of events we collectively call an inflammatory response. Inflammation literally means “to inflame.” It refers to the signs of inflammation, which are redness, heat, pain, and swelling. The histamine causes blood vessels near the site of the injury to become larger in diameter, which is called vasodilation. This allows more blood to flow through, which causes the heat and redness.

An inflammatory response is caused by the release of chemical messengers such as histamine and cytokines. The damaged cells in the skin trigger mast cells to release histamine, which causes vasodilation.

The histamine also triggers the blood vessels to become more permeable. This means that materials can pass into and out of the blood vessels more easily. Fluid from the blood is transferred to the tissue surrounding the injury, which leads to swelling. Additionally, cells called phagocytes migrate from the bloodstream to the injured area where they engulf any pathogens that may have entered the body.

Many types of immune cells are phagocytic, including neutrophils and macrophages. These phagocytic cells engulf pathogens by attaching to them using molecules on their cell surface and pulling them into the phagocytes’ interior. This forms a vesicle called a phagosome. The phagosome fuses with one or more lysosomes that are full of enzymes that break down the pathogen, destroying and neutralizing it. The remaining particles can be ejected from the cell by exocytosis. Some parts of the pathogen, called antigens, will be displayed on the cell surface to trigger further immune system actions.

Therefore, the process illustrated by cell X is phagocytosis.

Another cellular component of the innate immune response is a type of lymphocyte known as a natural killer cell, or NK cell. NK cells have the ability to recognize stressed, infected, or abnormal cells and eliminate them. Unlike the B and T lymphocytes, which are antigen specific and a part of the adaptive immune system, NK cells do not need to be activated by other immune cells or go through clonal selection where cells with more specificity toward an antigen are selected to produce more cells, a process that takes days, so they are able to respond rapidly to infection or to a growing tumor.

Another aspect of the innate immune system’s second line of defense is a series of proteins and their reactions known as the complement system. Complement enhances, or complements, the immune system’s various responses. It plays a role in both innate and adaptive immunity.

Complement proteins are always present in the blood. When they detect certain signs of infection, they work together with cytokines to activate a series of events called a complement cascade.

The complement cascade includes steps that aid in the progress of the inflammatory response, attracting and stimulating phagocytes that “eat” and destroy the pathogens. The complement system can also form a complex of chemicals that attacks and breaks apart the cell membrane of invading bacteria.

The second line of defense also includes some of the chemical messengers collectively known as cytokines. Cytokines function to facilitate communication between cells during an immune response.

An example of a cytokine functioning in the innate immune system is interferon. Interferon is a cytokine that is produced by host cells that are infected with a virus. Interferons interfere with virus replication in nearby host cells as a way of slowing the spread of infection. Interferons also stimulate the action of natural killer cells and macrophages. An illustration of the role of interferon is shown in Figure 6.

The chemical signals and cells involved in innate immunity are what we refer to as the second line of defense. These physiological responses are nonspecific, meaning they treat every pathogen the same way. Another way to say this is that they are not antigen specific.

After our skin is damaged by a cut or a scrape, inflammation works to clear any pathogens that may have been introduced. Normally, once any infection is cleared, cytokine and histamine levels drop and inflammation ends. However, this is not always the case and unregulated inflammation or immune function, such as autoimmune diseases, can lead to chronic inflammation.

Inflammation is an efficient and necessary immediate response to potential infection. However, inflammation can also be dangerous if left unregulated. Many autoimmune diseases, such as rheumatoid arthritis, lupus, and multiple sclerosis, have an inflammatory component where this system, which is designed to help our bodies, ends up attacking the body and causing injury instead.

Chronic inflammation, a low level of inflammation over a long period of time, has been linked to serious diseases like heart disease, type 2 diabetes, and cancer. Chronic inflammation has been shown to be caused by diets high in processed foods and destructive practices such as smoking and drinking alcohol. So, keeping your body healthy helps to keep your inflammation under control, which will prevent chronic disease.

It can take several days for the human adaptive immune system to mount a full response to an invading pathogen. During that time, the innate immune system is fighting off the infection by initiating inflammation like we have described. This inflammation, when occurring because of an infection somewhere besides the skin, can lead to the swelling, pain, and fever that we associate with the symptoms of many illnesses.

For example, a common infection of the tissue of the throat is one of several strains of streptococcus bacteria we call it “strep throat.” Within a couple of days of being exposed to these bacteria, the host will experience soreness and swelling in the throat as well as fever. As the bacteria multiply in the throat tissue, they cause injury to the cells there that release the histamine that initiates the inflammatory response.

The symptoms we commonly associate with the early signs of illness are actually our innate immune system doing its job to keep an infection under control until the adaptive immune system can mount a specific attack on the pathogen.

Our innate immune system has the tools to prevent most infections by either keeping pathogens out of the body or eliminating them in the early stages of infection. The adaptive immune system only kicks in when innate defenses prove inadequate. The relative times, durations, and strengths of protection of the various lines of immune defense are shown in the graph in Figure 7.

This two-tier system is highly efficient, preserving the resources of the adaptive immune system for the most severe cases only. Alternatively, the innate immune system keeps serious infections under control while the adaptive immune system prepares to step in and eliminate the threat.

How does the coronavirus enter the body, and what makes it so dangerous?

The coronavirus pandemic originated in China in late 2019 as a cluster of mysterious cases of pneumonia.

The culprit was found to be a new type of virus, now called severe respiratory syndrome coronavirus 2, or SARS-Cov-2.

The virus, and the disease it causes — COVID-19 — is fatal to a small percentage of the people it affects. Most of the danger is related to pneumonia, or an inflammation of the lungs caused by infection, although there is still much uncertainty about how the viral infection spreads and progresses in the body.

The early symptoms are usually a cough, fever and shortness of breath, and look a lot like the flu or common cold. Symptoms appear 2-14 days after exposure, the Centers for Disease Control said.

The virus circulates through droplets in the air spread by coughing or sneezing, and enters the body through the mouth, eyes or nose. It can remain viable on surfaces for hours to days, and may be able to enter the lungs directly when inhaled.

After entering the body, the virus spreads to the back of the nasal passage and to mucous membranes in the throat, attaching to the body’s cell receptors.

The viral particles hook onto the outer walls of the host’s cells, the virus’s genetic material breaches the cell membrane, and it then hijacks the cell into making more copies of the virus. The virus copies proliferate, break out of the cell, and infect other cells in the body. A single cell can churn out millions of copies of the virus before it dies.

Into the lungs

The virus then moves from the back of the throat down the bronchial tubes toward the lungs, according to a report from The New York Times based on interviews with leading experts in infectious diseases.

The infection can then reach the lungs, causing inflammation in their mucous membranes and damaging their air sacs. The inflammation hampers the lungs’ ability to oxygenate the blood and remove carbon dioxide from the bloodstream.

The inflammation in the lungs, and their reduced efficacy, can cause them to fill with fluids, pus and dead cells, and cause an infection, leading to pneumonia.

Some people who are infected have difficulty breathing and require a ventilator, and for others, the lungs become so inundated with fluids that even with intervention, they die.

People suffering from pneumonia are also at a heightened risk for secondary viral and bacterial infections.

Pneumonia caused by the coronavirus appears to be more severe than most cases of the disease, and affects a greater portion of the lungs, experts told The Guardian newspaper. Pneumonia is usually caused by bacterial infection, which can be treated with antibiotics.

The COVID-19 lung infection appears to start on the outer parts of the sides of the lung, then moves to more central areas, including the upper respiratory tract and trachea.

The virus can also enter the bloodstream, and may be able to infect the gastrointestinal system, causing symptoms like diarrhea and indigestion. The infection can also directly damage organs including the heart, kidneys, and liver, and cause bone marrow to become inflamed. Small blood vessels may also be vulnerable to inflammation.

The body’s own immune response to the infection can cause inflammation and organ malfunction. It is still unclear if the brain is affected.

The Centers for Disease Control said the virus has been detected in blood and stool specimens, but it is unclear whether it can spread through any bodily fluids.

There are at least six other kinds of coronavirus that can infect humans. Some cause colds, while others caused the SARS and MERS outbreaks. The SARS-Cov-2 strain is believed to have originated in bats.

Health experts recommend regular hand washing to limit exposure. Soap effectively destroys the virus when it is outside the body because it dissolves the virus’s lipid bilayer, or its outer membrane made of fat, causing the viral particles to fall apart. Hand sanitizer and alcohol products are less effective than soap in combating the virus.

Who is most at risk?

Over 60,000 people have recovered from the virus, at the time of this writing. The World Health Organization says about 80% of people recover without any treatment from specialists, and about one in six become seriously ill.

Older people and those with underlying health problems are the most vulnerable. The immune system tends to weaken with age, even in healthy and active people.

Risk from coronavirus starts to increase for people who are over 60 and is heightened more for those over 80. Longtime smokers are likely more susceptible to a serious infection due to their damaged lungs.

According to the World Health Organization, people with mild illness recover in about two weeks, while those with more severe ailments who survive may take three to six weeks to rebound.

Worldwide, there have been over 134,000 confirmed cases of the coronavirus and over 5,600 deaths since it first emerged in China in December, according to AFP figures on March 14.

The WHO has estimated that the virus will kill around 3.4% of patients.

Various reports quoted by the Associated Press have estimated the fatality rate from less than 1% to as high as 4% among cases diagnosed so far, depending on location.

An official from the Centers for Disease Control estimated that in the US, between 0.25% and 1% of patients would die.

The World Health Organization has declared the virus a pandemic and warned, also on March 14, that it was “impossible” to know when it would peak.

As of this writing, there have been 143 confirmed coronavirus cases in Israel, with three people in serious condition. There have not been any deaths from the virus. Some 35,000 Israelis have entered quarantine.

Israel has taken a number of stringent measures to curb the spread of the virus, including banning public gatherings of over 100 people and closing schools and universities, with stricter measures likely to follow.

All Israelis returning from overseas are required to quarantine at home for 14 days. Non-Israeli nationals were barred from entering the country as of Thursday at 8 p.m., unless they can demonstrate an ability to self-quarantine for two weeks.

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The 4-Stage Process Of Wound Healing: Making Skin Stronger Than Before

What’s the largest organ in your body? As a kid that might have gotten you it’s your skin, which serves as a layer of protection between your inner tissues and the outside world.

Because of its protective function, your skin must have an action plan for healing itself when you get hurt, no matter if it's scratched, bruised, or wounded. The body’s self-healing properties are fascinatingly harmonious and rather beautiful, as the video below depicts. Various cells and mechanisms work together as though forming a puzzle, rebuilding your layers of skin.

When the top layer — the epidermis — is broken by a light scratch, not much occurs. You might see some dead skin cells flake off. But when something cuts into the deeper, next layer — called the dermis — you’ll see blood, and your body triggers a four-stage process of healing itself. The skin has to respond to two major threats at first: the loss of blood, and the lack of a physical barrier (the epidermis) between your innards and the outer world. An open cut is an open doorway to bacteria and other pathogens, far more vulnerable to infection, so the body must act quickly to regenerate the epidermis.

First, red blood cells form a blood clot, which helps stop the bleeding and creates a temporary barrier that prevents pathogens from getting into the open wound. A few hours later, your skin might turn red and look swollen. This is the inflammation phase, when the body sends white blood cells to capture and fight off any rogue bacteria that happened to get through. Next, fibroblast cells enter the wound, dropping off collagen, which forms connective skin tissue to replace what was there before. The dermis and epidermis, then connect and contract to close the wound. After this full process, the skin is likely to be much stronger than it was before the wound. To see the entire process, watch the video below.

Review Questions

Which of the following cells is phagocytic?

Which structure allows lymph from the lower right limb to enter the bloodstream?

  1. thoracic duct
  2. right lymphatic duct
  3. right lymphatic trunk
  4. left lymphatic trunk

Which of the following cells is important in the innate immune response?

Which of the following cells would be most active in early, antiviral immune responses the first time one is exposed to pathogen?

Which of the lymphoid nodules is most likely to see food antigens first?

  1. tonsils
  2. Peyer&rsquos patches
  3. bronchus-associated lymphoid tissue
  4. mucosa-associated lymphoid tissue

Which of the following signs is not characteristic of inflammation?

Which of the following is not important in the antiviral innate immune response?

Enhanced phagocytosis of a cell by the binding of a specific protein is called ________.

Which of the following leads to the redness of inflammation?

  1. increased vascular permeability
  2. anaphylactic shock
  3. increased blood flow
  4. complement activation

T cells that secrete cytokines that help antibody responses are called ________.

The taking in of antigen and digesting it for later presentation is called ________.

Why is clonal expansion so important?

  1. to select for specific cells
  2. to secrete cytokines
  3. to kill target cells
  4. to increase the numbers of specific cells

The elimination of self-reactive thymocytes is called ________.

Which type of T cell is most effective against viruses?

Removing functionality from a B cell without killing it is called ________.

Which class of antibody crosses the placenta in pregnant women?

Which class of antibody has no known function other than as an antigen receptor?

When does class switching occur?

Which class of antibody is found in mucus?

Which enzymes in macrophages are important for clearing intracellular bacteria?

What type of chronic lung disease is caused by a Mycobacterium?

Which type of immune response is most directly effective against bacteria?

What is the reason that you have to be immunized with a new influenza vaccine each year?

  1. the vaccine is only protective for a year
  2. mutation
  3. macrophage oxidative metabolism
  4. memory response

Which type of immune response works in concert with cytotoxic T cells against virally infected cells?

Which type of hypersensitivity involves soluble antigen-antibody complexes?

What causes the delay in delayed hypersensitivity?

  1. inflammation
  2. cytokine release
  3. recruitment of immune cells
  4. histamine release

Which of the following is a critical feature of immediate hypersensitivity?

  1. inflammation
  2. cytotoxic T cells
  3. recruitment of immune cells
  4. histamine release

Which of the following is an autoimmune disease of the heart?

What drug is used to counteract the effects of anaphylactic shock?

Which of the following terms means &ldquomany genes&rdquo?

Why do we have natural antibodies?

  1. We don&rsquot know why.
  2. immunity to environmental bacteria
  3. immunity to transplants
  4. from clonal selection

Which type of cancer is associated with HIV disease?

How does cyclosporine A work?

  1. suppresses antibodies
  2. suppresses T cells
  3. suppresses macrophages
  4. suppresses neutrophils

What disease is associated with bone marrow transplants?