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Human Defense Against Infection

The ubiquity of microorganisms presents a considerable challenge to the bodily defense mechanisms of all living things. As a consequence, it is necessary for large, complex organisms such as humans to have at their disposal a multilayered system to prevent infection and colonization by potential and real disease causing agents. This system is composed of three lines: (1) nonspecific defenses against microorganisms, air and fluid-borne particles, composed of skin, mucus membranes, cellular and chemical means of limiting and preventing infection, (2) humoral (body fluid) defenses which attack molecules or parts of the structure of microorganisms, and (3) cell-mediated defenses, which target whole cells expressing molecular components that are foreign to the body.

Nonspecific Defenses

Skin

Nonspecific body defenses do not target molecules, individual cells or cell parts. Instead, they act as a barrier, providing a broad coverage against invasion of the body by any foreign particle. The first and largest of these defenses is the skin, a cutaneous membrane which covers the external portion of the human body. Skin is composed of an outermost portion, the stratum corneum, the cells of which are dead and filled with the protein keratin. Between 15 and 20 layers of dead cells make up this layer, which is exposed to external air and fluids. Beneath this portion lie several other layers, the stratum lucidum, stratum granulosum, stratum spinosum, and the stratum germativium (stratum basale; the germinating layer where new living cells divide), anchored to the underlying dermis. Living cells from the stratum germativium migrate through the other layers where they die and are filled with keritin. About two weeks after the dead cells reach the surface of the skin, they are sloughed (shed) away mechanically, carrying with them any transient microbes lying on the outermost surface. As long as this layer remains intact, it provides an effective barrier against the entry of potential pathogens into the body.

Associated with the skin are several accessory structures which aid in this external protection. Hair, especially that guarding the entrance to the nostrils and external ear canals, prevents entry of foreign particles. Sebaceous (oil) glands along the skin and associated with hairs secrete sebum, which is composed of water, triglyceride fatty acids, salts, proteins, and cholesterol, and is bacteriostatic. Ceruminous (wax) glands associated with the external ear canal excrete secretions which mix with sebum to form cerumin or ear wax, which traps bacteria and also inhibits their growth. Sweat glands secrete a combination of water, sodium chloride, and metabolic waste products which inhibit the growth of non-normal microflora (some normal flora thrive on certain sweat secretions such as those produced by the apocrine sweat glands associated with hairs of the armpits and groin of adults and pubescent teens, and their metabolism of these compounds intensifies body odor associated with such bodily areas).

Epithelial and Mucus Membranes

Mucus membranes are layers of epithelial tissue lining the outer surfaces of the respiratory and gastrointestinal tracts. The lining of the upper respiratory tract is composed of epithelial cells of two main types: goblet cells, which produce and secrete mucus, a sticky compound which traps dust and microbes, and ciliated epithelium, which pushed the secreted mucus along. The combination of mucus producing goblet cells and ciliated epithelium forms the mucociliary escalator, which moves forein particles away from the lower respiratory tract, toward the oropharynx, where it can be swallowed. The mucus-secreting layers of the gastrointestinal tract trap microbes so they can be destroyed by chemical, cellular, and humoral defenses, and the lining of the urinary tract is bathed in urine, an acidic waste material which limits bacterial growth.

Chemical Defenses

Nonspecific chemical defenses are both secreted on the surface of the epithelium and are found in body fluids. Lysozymes are enzymes found in lacrimal (tear) fluid which bathes the epithelial surface of the eyes, as well as in saliva, nasal secretions, and sweat. These lyse the cell wall of bacterial cells. Mucus, sebum, and sweat all inhibit bacterial growth. Urine in the urogenital tract, and hydrochloric acid in the stomach maintain an acidic environment which is not tolerable to most microorganisms. Histamines and leukotrienes, amine and lipid compounds secreted by mast cells and basophils, increase vascular permiability, stimulate glandular secretions, cause contraction of smooth muscle, and attract eosinophils. Kinins, polypeptides synthesized from plasma proteins stimulate pain receptors and attract neutrophils. Prostaglandins, ring- shaped lipid molecules, also stimulate pain receptors, cause smooth muscle contraction, vasodilation, and increase vascular permiability. Interferon, a protein compound produced by cells which have been invaded by viruses, blocks the transcription of new viral early proteins to prevent the infection of healthy cells. Endogenous pyrogens are compounds produced by eosinophils, monocytes, and other cells which induce fever, the raising body temperature to inhibit bacterial growth through the denaturing of their metabolic proteins. Complements, a series of nine plasma proteins, increase vascular permiability, activate kinins, chemically attract phagocytic cells, stimulate the release of histamines, and nonselectively lyse bacterial cells.

Cellular Defense

Leukocytes are cells derived from undifferentiated hemocytoblasts in bone marrow or undifferentiated stem cells present in lymphatic tissues. Granulocytes, such as motile basophils store chemical mediators such as kinins, prostaglandins, and histamines in small membrane-bound packages which can be released into the surrounding environment via a process called degranulation. Some of these cells, such as neutrophils, have the ability to phagocytize foreign substances,and bacteria. These are motile via ameoboid movement, and can squeeze between cells through a process called diapedesis. During phagocytosis, the leukocyte wraps a portion of its plasma membrane around a foreign substance such as a bacterial cell, forming an internal vacuole. The vacuole is contacted by and incorporates itself with a lysosome, becoming a phagolysosome. Compounds such as lysozyme, hydrogen peroxide, and superoxide attack the engulfed cell, breaking it down. Some of the digested material can pass into the cytoplasm of the phagocyte, while the waste is released from the cell via exocytosis. Eosinophils enter tissues from the bloodstream and release chemical agents which inhibit the release of histamine and other inflammatory compounds, and also attack invading helminth (worm) parasites. Agranulocytes are cells which do not have granules associated with their cytoplasm. These include B plasma and memory cells, and T lymphocytes associated with the specific immune response.

Examples of nonspecific cells other than granulocytes or agranulocytes include phagocytic microglia of the central nervous system, and wandering macrophages derived from monocytes which travel through the bloodstream and lymphatic tissues, removing bacteria, foreign particles, and dead cells from the body. Fixed cell types include granular mast cells associated with connective tissues, which release chemical mediators through degranulation, fixed macrophages such as dendritic cells of the dermis, Kuppfer cells of the liver, and alveolar macrophages (dust cells) associated with the alveoli of the lungs. Motile leukocytes are attracted to the site of injury or microbial invasion by chemotactic factors such as complements, leukotrienes, kinins, and histamines.

Normal Microflora

One nonspecific defense mechanism is not derived from specific human cell types, but from the presence of microorganisms which are always associated with the body. Normal microflora (normal flora) are organisms which live in and on the body of the human, most usually as commensals. The healthy human body serves as the home for many species of bacteria, protists, and some fungi, which maintain their place through the competitive exclusion of transient and possibly pathogenic microorganisms. While some of these can become opportunistic pathogens, their influence is usually benign to their own individual host unless the normal homeostatic balance of the host has been upset by disease, stress, exposure, or improper use of antibiotics. Examples of normal microflora include Staphylococcus epidermidis and the diptheroid bacteria associated with the skin such as Corynebacterium xerosis, a hemolytic streptococci, Veillonella sp., Peptostreptococcus sp., and small amoebas found in the oral cavity, Fusobacterium sp., Bacteroides sp., Escherichia coli, lactobacilli, amoebas, and fungi such as Candida albicans, all found in the gastrointestinal tract, streptococci , lactococci, and lactobacilli such as Lactobacillus acidophilis in the vaginal canal. The lower respiratory tract and the upper urogenital systems in both healthy males and females are generally germ-free.

Inflammation

If the body is invaded by potential pathogens either through the parenteral, oral, or airborne routes, one of the major mechanisms in nonspecific defense involves a swelling of the afflicted and surrounding tissues called inflammation. Inflammation can be initiated by host cell damage, which releases chemical mediators such as histamines, bradykinin, and prostaglandins into the area surrounding the site of injury. Histamines and prostaglandins trigger increased vascular permiability resulting in the loss of fluid into the tissue at the injury site (edema), vasodilation of the small blood vessels near the site which results in increased bloodflow, redness and excess heat, and chemotaxis of neutrophils. Bradykinin and prostaglandins also lower the action potentials necessary for the firing of pain receptors near the injured area. If the tissue injury damages capillaries (as is the case in parenteral injuries), blood flows into the wound and begins to clot as the blood protein fibrinogen is converted to fibrin, a sticky, fibrous filaments which form a mat, trapping red blood cells to seal the injured area. Over time, germinal cells of the stratum germativium will begin to replace the damaged tissue and the clotted mat, now called a scab, will fall off. At the same time, connective fibers in the dermis begin to fill in the damaged are beneath the new epidermis, and as capillaries penetrate this, granulation tissue is formed (excess granulation tissue just beneath the new epidermis of a wound is what causes scars to form). If microbes are still present, neutrophils which have travelled to the injury site via the bloodstream and entered the site through diapedisis begin to phagocytize them. The combination of dead bacteria, neutrophils, fluid, and damaged cell parts compose pus, a sticky, whitish compound. Pyogenic (pus-forming) activity is associated with puncture wounds, pimples and boils which occur when pores of the skin are blocked by dirt and oil, providing an amenable environment for skin-borne staphs, streps, and Propionibacterium acnes to successfully colonize the site as opportunistic pathogens, infection of the throat by b hemolytic group A Streptococcus pyogenes, peritonitis (invasion of the peritoneal cavity by flora of the gastrointestinal tract), bacterial meningitis caused by Nesseria meningitidis, and other systemic and localized infections. In time, wandering macrophages will enter the site to phagocytize the damaged tissue and dead cells, but in some instances, excess pus must be drained before the healing process can continue.

Humoral Defense

The second line of defense against infection specifically targets molecules called antigens which express themselves on the surface of cells or are present in bodily fluids. The portion of an antigen molecule which is targeted by humoral defenses is called an antigenic determinant, or epitope. All microbes, human tissue and human cell types express various antigens, but in the healthy individual, the body exhibits immunological tolerance, meaning that it has the ability to discriminate between "self" and "nonself" antigens. This discrimination is probably due to the presence of self-recognition glycoproteins called major histocompatibility complexes (MHCs) and is important, since it prevents the attack on an individual's own cells and tissues. If tolerance is lost, autoimmune disease can result.

The molecules responsible for humoral immunity to disease are called immunoglobulins, also known as antibodies. Antibodies are proteinaceous cell markers which differ from other receptor site proteins, in that they have the ability to detach themselves from the cells which produce them and circulate independently throughout the fluid compartments of the body. Some have the ability to be secreted across membranes, which allows them to confer protection to external body surfaces, or to pass through the placental barrier to provide protection to a developing fetus, while others are relatively large and incapable of migrating out of the bloodstream.

The basic immunoglobulin structure consists of four protein chains, two light chains and two heavy chains, bound to one another by sulfide bonds. At the distal end of the light/heavy chain pairs are areas called variable regions, which have an affinity for specific antigenic determinants. Antibodies which are unbound to antigens have a Y-shape, due to the conformation of light and heavy protein chains, but these can assume a T-shape when attached to two separate antigens expressed by two separate cells or particles by bending at the hinge region of the molecule. At the base of the heavy chains is an area called the constant (Fc) region which can bind to other antibodies, or serve as a binding site for the receptor proteins on a wandering macrophage, mast cell, basophil, or lymphocyte. During the phenomenon called opsonization, an antibody which has attached itself to an antigenic determinant can "lead" a macrophage to the cell or molecule expressing the antigen and facilitate phagocytosis. Some antibodies also have complement binding sites, which enhances their ability to inactivate and kill foreign cells.

There are five major classes of immunoglobulins:

Immunoglobulin G (IgG) is found in the greatest concentration in the blood and other fluids of the body, accounting for over 80% of all of the circulating immunoglobulins. IgG has two variable regions, complement-binding sites, and can promote opsonization. It functions as an agent of agglutination (clumping of cells or particles bound by antibodies), precipitation (causing clumped antigens to condense and fall out of serum), complement fixation and cell lysis, and as an antitoxin, by binding to exotoxin epitopes to neutralize them. It can cross the placental barrier and is present in colostrum and breast milk, so it provides the developing fetus and newborn infant with a measure of immune protection.

Immunoglobulin A (IgA) composes about 15% of the circulating antibodies in the system. IgA is a dimer, composed of two Y-shaped molecules joined by a secretory protein which allows it to be secreted on the surface of the epithelium. It can be found in saliva, tears, colostrum, breastmilk, and mucus, and functions in agglutination and precipitation.

Immunoglobulin E (IgE) has a structure similar to that of IgG, but is produced in extremely small quantities (0.002%). This immunoglobulin is often called reagin, because its primary function is in the elliciting the degranulation of mast cells and basophils in immediate-type hypersensitive reactions. This form of hypersensitive reaction occurs when small antigen molecules called allergens bind to the variable region of IgE when the antibody is concurrently bound by its Fc region to to a receptor site on a mast cell or basophil. The formation of this allergen-antibody-cell complex triggers degranulation and the release of histamines and leukotrienes which mediate the inflammatory response, which in this case generally exhibits itself as sneezing, watery eyes, running nose, or uticaria (hives).

Immunoglobulin D (IgD) is also a single molecule, and is found in very small (0.2%) concentrations. The real functions of IgD are not well understood, but since these molecules are generally observed to be bound to cells, it has been hypothesized that their function involves the regulation of the differentiation of immune cell types.

Immunoglobulin M (IgM) is the largest of the immunoglobulins, composing about 5% of the total quantity of circulating antibodies, and is the first circulating antibody to appear in the bloodstream following an initial infection, though its titer (concentration) soon drops as more IgG appears. It is a pentamer, composed of five subunits having the same basic structure as IgG. Because of its large size, IgM cannot cross membrane barriers, so it remains in the bloodstream. IgM functions in precipitation, agglutination, complement fixation and lysis,and opsonization. Since it has ten variable sites, IgM has very strong agglutination properties and is used in such in- vitro serological tests such as blood typing.

Immunoglobulins are produced by lymphocytes called B cells (the name comes from a structure called the Bursa of Fabricus located just superior to the cloaca of birds such as chickens). B cells, like all other formed elements (cells) of the blood, are derived from stem cells called hemocytoblasts in red bone marrow. These migrate to the lymphatic system to lymph nodes where they mature. On the surface of each pre-B lymphocyte are receptor sites composed of four polypeptide chains with two variable regions for the binding of antigenic determinants on the surface of antigens. While it is possible for some B cells to bind directly to antigens, most antigenic determinants must be first processed in the cytoplasm, then expressed on the outer surface of special antigen-presenting cells (APCs), such as wandering macrophages, fixed dendritic cells, and other B cells. APCs express processed antigens bound to their MHC proteins, where they deliver them to B cell receptors. There are two classes of MHC protein; MHC class I proteins bind antigens produced in cells, such as the early proteins of viral replication, and MHC class II proteins bind antigens made outside of cells, such as proteins produced by bacteria. APCs also produce chemical agents called interleukins, which stimulate division of activated lymphocytes.

When the cell comes in contact with a new antigen, it responds by differentiating into two types; B plasma cells which produce antibodies specific to the particular antigen and release these into the system, and B memory cells which do not produce antibodies, but retain the genetic memory of how to make the same specific type of antibody. B memory cells can circulate in the bloodstream or reside in the lymphoid tissue for long periods of time, until they are presented with the same antigen.

When this occurs, they differentiate and clone new plasma and memory cells, increasing the titer of circulating antibodies much more rapidly than when the original pre-B cell was first introduced to the antigen. If the concentration of antibodies is graphed over time for both differentation events, patterns emerge. These patterns are called the Primary and Secondary Responses to infection by the same antigen. Upon the first contact with a new antigen, the primary response occurs. The bulk of circulating antibodies produced are IgM, with fewer IgG. This response peaks fairly rapidly, then subsides. On the second exposure to the same antigen, the secondary response begins as memory cells differentiate, with the titer of circulating antibodies increasing markedly over that of the primary response. The bulk of produced antibodies in the secondary response, however, are IgG, with fewer IgM. Any additional contact with the same antigen, called shocking doses, triggers even greater antibody titers, insuring that the antigen is quickly neutralized. The reason all reactions to the same antigen following initial exposure are so rapid is called the anamnestic response, meaning the ability of the B memory cell to retain genetic information.

Cell-Mediated Defense

While the cells responsible for humoral immunity produce immunoglobulins which target specific molecules, those associated with cell-mediated immunity coordinate immune activity and target whole cells expressing foreign antigens. These cells also are derived from hemocytoblasts in red bone marrow and, like B cells, travel to lymphatic tissue, but they first migrate to a small mass of glandular tissue called the thymus. There these pre-T cells are stimulated by hormones such as thymosin and thymopoetins to become mature T cells. T cells, like B cells, have antigen- binding receptor sites, however, these site on T cells are composed of two polypeptide chains, subdivided into variable and constant regions.

Several cell types are responsible for the cell-mediated immune response.These can be separated from one another on the basis of a group of peripheral proteins on the cell membrane called cluster of differentiation (CD) markers. Natural killer cells (N-K cells) are not Tor B lymphocytes and have no CD markers. They target and destroy host cells which have been invaded by viruses, so they are generally considered to be part of the nonspecific defense against infection. N-K cells can locate these specific cells, since they usually express viral antigens on their cell membranes once infection has occurred. CytotoxicT cells have CD8 markers and kill any cell expressing foreign antigens, such as cells which have been invaded by viruses, cellular parasites, and cancer cells. They also produce lymphokines such as perforin, which lyse target cells, and interferon to block transcription of viral mRNA. T memory cells retain genetic information about specific antigens. They, like B memory cells, can differentiate into other T cell types upon exposure to an antigen delivered by an APC. Delayed-type hypersensitivity cells (DHC) have CD4 antigens and produce chemotactic compounds and lymphokines such as macrophage arming factor (MAF) and specific macrophage activating factor (SMAF) which lead macrophages to the site of an antigen and elicit inflammation. The activity of DHC cells is responsible for delayed hypersensitive reactions such as contact dermatitis caused by exposure to chemicals such as urushiol found in the leaves of plants such as poison ivy (Toxicodendron radicans), soaps, detergents, and makeup. T helper and T suppressor cells regulate the activities of other specific immune cell types. T helper cells have CD4 antigens, and they activate B and T lymphocytes, while T suppressors have CD8 antigens and suppress or slow lymphocyte activity, thus reducing the risk of potential autoimmune disease.

Types of Immunity

Immunity to disease is acquired over time, and is classified into two distinct categories, natural, meaning that response to antigens occurs as part of the natural environment to which the individual is exposed, and artificial, meaning that the responseis triggered by deliberate exposure of the individual to an antigen or antigens. Both of these can be further subdivided as active, wherein the individual's own body provides an immune response, or passive, where the immune activity such as the production of antibodies began in an outside source, such as an animal or another human, then transferred to the individual. Naturally acquired active immunity develops as a consequence of an individual's own exposure to new antigens, which triggers the immune system to respond. After initial exposure to an antigen, the primary immune response occurs, and it is at this time that the individual suffers from the effects of a disease and become contaigious if the microorganism responsible is a pathogen. Naturally acquired passive immunity results from the transfer of antibodies across the placenta and in colostrum and breast milk, along with monocytes, from mother to infant. Artificially acquired active immunity is produced by the artificial introduction of antigens into the body of an individual. Artificially acquired passive immunity is produced through the introduction of antibodies produced by an outside source into the body of an individual.

Both forms of artificial immunity are established via vaccination. Vaccine types include:

1. Attenuated vaccines, produced by removing the virulence factors of an microorganism, then injecting the active agent into the body. Examples include the vaccinations for small pox, polio (Sabin oral form), measles, mumps, and yellow fever.

2. Killed or inactivated vaccines such as polio (Salk injection), influenza A virus, rabies, cholera (Vibrio cholera), bubonic plague (Yersinia pestis), typhoid fever (Salmonella typhi), and MMR (measles, mumps and rubella viruses).

3. Recombinant vaccines produced by genetic engineering, such as the Hepatitis B vaccine.

4. Toxoid (antitoxin) vaccines composed of denatured toxins, including those diptheria (Coynebacterium diptheriae), pertussis (Bordetella pertussis), and tetanus (Clostridium tetani; combined they are called DPT), spiders, snakes, scorpions, and centipedes.

5. Component vaccines composed of parts or fragments of cells containing specific antigens, such as pneumovax (23 capsular antigens of different strains of Streptococcus pneumoniae), meningiococcal meningitis vaccine (Neisseria meningitidis) and Hib (Haemophilis influenzae).

Cooperative Learning Activities

I. Fill in the Blanks

1. Skin, mucus membranes, and chemical defenses all make up ______________ defenses
against infection.

2. __________ defenses are based in the body fluids, and are due to the activity of antibodies.

3. Whole, infected cells are destroyed by the ___________ __________ immune response.

4. The living layer of cells which gives rise to the epidermis is called the _____________.

5. Two external chemical agents which limit bacterial growth are __________ and __________.

6. __________ cell produce mucus which lines the external surface of the upper respiratory tract.

7. The combination of mucus-producing cells and cilialted epithelium of the upper respiratory tract
is called the __________ __________.

8. __________ are enzymes found in lacrimal fluid, saliva, nasal mucus, and sweat.

9. __________ are synthesized from plasma proteins and serve to stimulate pain receptors.

10. Eosinophils and monocytes release __________ __________ which raise body temperature.

The major formed elements of blood, including red blood cells and leukocytes are derived from undifferentiated

_______________ found in red bone marrow. The two major categories of leukocytes are __________ and __________.

Granulocytes such as __________ release chemical mediators of inflammation, such as kinins, prostaglandins, and

__________ which trigger vasodilation and smooth muscle contraction, while __________, the most common type of white

blood cell, are actively phagocytic and can undergo diapedesis to reach a site of injury from the bloodstream. Agranulocytes

include __________ cells which actively secrete antibodies, and __________ cells, which function in the cell-mediated

immune response.

II. Critical Thinking

1. An individual picks up a splinter in her foot. Describe the process of inflammation which occurs
after this injury.

2. Compare and contrast the terms antigen and allergen.

3. What is immunological tolerance, and why is it important to the body's defense against infection?

4. In the primary immune response, more IgM is produced than IgG, but in the secondary response,
the opposite is true. Why is this so?

5. How does the HIV virus suppress the activity of the entire immune system?

6. Why do hypersensitivity reactions occur?