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The internal environment of animals provides attractive conditions for the growth of bacteria, viruses, and other organisms. Although some of these organisms can live symbiotically within animals, many either cause destruction of cells or produce toxic chemicals. To protect against these foreign invaders, humans possess three levels of defense.

The skin and mucus membranes provide a nonspecific first line of defense against invaders entering through the skin or through openings into the body. A nonspecific defense is not specialized for a particular invader. Rather, it is a general defense against all kinds of pathogens. The first line of defense features the following characteristics:

1. Skin is a physical and hostile barrier covered with oily and acidic (pH from 3 to 5) secretions from sweat glands.

2. Antimicrobial proteins (such as lysozyme, which breaks down the cell walls of bacteria) are contained in saliva, tears, and other secretions found on mucus membranes.

3. Cilia that line the lungs serve to sweep invaders out of the lungs.

4. Gastric juice of the stomach kills most microbes.

5. Symbiotic bacteria found in the digestive tract and the vagina outcompete many other organisms that could cause damage.

The second line of defense involves several nonspecific mechanisms, as follows:

1. Phagocytes are white blood cells (leukocytes) that engulf pathogens by phagocytosis.

They include neutrophils and monocytes. Monocytes enlarge into large phagocytic cells called macrophages. Other white blood cells called natural killer cells (NK cells) attack abnormal body cells (such as tumors) or pathogen-infected body cells.

2. Complement is a group of about twenty proteins that “complement” defense reactions. These proteins help attract phagocytes to foreign cells and help destroy foreign cells by promoting cell lysis (breaking open the cell).

3. Interferons are substances secreted by cells invaded by viruses that stimulate neighboring cells to produce proteins that help them defend against the viruses.

4. The inflammatory response is a series of nonspecific events that occur in response to pathogens. When skin is damaged, for example, and bacteria or other organisms enter the body, the following events occur:

• Histamine is secreted by basophils, white blood cells found in connective tissue.

• Vasodilation (dilation of blood vessels), stimulated by histamine, increases blood supply to the damaged area and allows for easier movement of white blood cells (and other body fluids) through blood vessel walls. This also causes redness, an increase in temperature, and swelling. The increase in temperature, like a fever, may stimulate white blood cells, and they may make the environment inhospitable to pathogens.

• Phagocytes, attracted to the injury by chemical gradients of complement, arrive and engulf pathogens and damaged cells.

• Complement helps phagocytes engulf foreign cells, stimulate basophils to release histamine, and help lyse foreign cells. The immune response is the third line of defense. It differs from the inflammatory response in that it targets specific antigens. An antigen is any molecule, usually a protein or polysaccharide, that can be identified as foreign. It may be a toxin (injected into the blood by the sting of an insect, for example), a part of the protein coat of a virus, or a molecule unique to the plasma membranes of bacteria, protozoa, pollen, or other foreign cells.

The major histocompatibility complex, or MHC, is the mechanism by which the immune system is able to differentiate between self and nonself cells. The MHC is a collection of glycoproteins (proteins with a carbohydrate) that exists on the membranes of all body cells. The proteins of a single individual are unique, originating from twenty genes, each with more than fifty alleles each. Thus, it is extremely unlikely that two people, except for identical twins, will possess cells with the same set of MHC molecules. The primary agents of the immune response are lymphocytes, white blood cells (leukocytes) that originate in the bone marrow (like all blood cells) but concentrate in lymphatic tissues such as the lymph nodes, the thymus gland, and the spleen. The various kinds of lymphocytes are grouped as follows:

1. B cells. These are lymphocytes that originate and mature in the bone marrow (remember B cell for bone). B cells respond to antigens. The plasma membrane surface of B cells is characterized by specialized antigen receptors called antibodies. Antibodies have the following properties:

• Antibodies are proteins.

• Each antibody is specific to a particular antigen.

• There are five classes of antibodies (or immunoglobulins): IgA, IgD, IgE, IgG, IgM. Each class is associated with a particular activity.

• Each class of antibodies is a variation of a basic Y-shaped protein that consists of constant regions and variable regions. The variable regions are sequences of amino acids that differ among antibodies and give them specificity to antigens.

• Antibodies inactivate antigens by binding to them. Inactivation is followed by macrophage phagocytosis. In addition, by binding to surface antigens of nonself cells, antibodies stimulate complement proteins to bring about the lysis of pathogens. When B cells encounter antigens that specifically bind to their antibodies, the B cells proliferate, producing two kinds of daughter B cells, as follows:

• Plasma cells are B cells that release their specific antibodies which then circulate through the body, binding to antigens.

• Memory cells are long-lived B cells that do not release their antibodies in response to the immediate antigen invasion. Instead, the memory cells circulate in the body and respond quickly to eliminate any subsequent invasion by the same antigen. This mechanism provides immunity to many diseases after the first occurrence of the disease.

2. T cells. T cells are lymphocytes that originate in the bone marrow, but mature in the thymus gland (T cell for thymus). Like B cells, the plasma membranes of T cells have antigen receptors. However, these receptors are not antibodies, but recognition sites for molecules displayed by nonself cells. Self and nonself cells are distinguished as follows:

• The MHC markers on the plasma membrane of cells distinguish between self and nonself cells.

• When a body cell is invaded by a virus, by a foreign cell, or by any antigen, the body cell displays a combination of self and nonself markers. T cells interpret this aberrant display of markers as nonself.

• Cancer cells or tissue transplant cells, or other cells that display aberrant markers, are recognized as nonself cells by T cells. When T cells encounter nonself cells, they divide and produce two kinds of cells, as follows:

• Cytotoxic T cells (or killer T cells) recognize and destroy nonself cells by puncturing them, thus causing them to lyse.

• Helper T cells stimulate the proliferation of B cells and cytotoxic T cells. When an antigen binds to a B cell or when a nonself cell binds to a T cell, the B cell or T cell begins to divide, producing numerous daughter cells, all identical copies of the parent cell. This process is called clonal selection, since only the B or T cell that bears the effective antigen receptor is “selected” and reproduces to make clones, or identical copies of itself. Clonal selection results in a proliferation of B cells and T cells that will engage a specific, invading antigen. The responses of the immune system are categorized into two kinds of reactions, as follows:

1. The cell-mediated response uses mostly T cells and responds to any nonself cell, including cells invaded by pathogens. When a nonself cell binds to a T cell, the T cell undergoes clonal selection, initiating the following chain of events.

• T cells produce cytotoxic T cells. These cells destroy nonself cells.

• T cells produce helper T cells.

• Helper T cells bind to macrophages. Macrophages that have engulfed pathogens display aberrant plasma membrane markers. Helper T cells identify these marker combinations as nonself and bind to these macrophages.

• Helper T cells produce interleukins to stimulate a proliferation of T cells and B cells. When helper T cells bind with macrophages, they release interleukins, or communication chemicals “between leukocytes.” The interleukins initiate a sequence of positive-feedback events that result in the proliferation of interleukins, macrophages, helper T cells, B cells, and cytotoxic T cells.

2. The humoral response (or antibody-mediated response) involves most cells and responds to antigens or pathogens that are circulating in the lymph or blood (“humor” is a medieval term for body fluid). It includes the following events:

• B cells produce plasma cells. The plasma cells, in turn, release antibodies that bind with antigens or antigen-bearing pathogens.

• B cells produce memory cells. Memory cells provide future immunity.

• Macrophage and helper T cells stimulate B cell production. In many cases, the antigen will not directly stimulate the proliferation of B cells. Instead, the antigen or antigen-bearing pathogen must first be engulfed by a macrophage. T cells then bind to the macrophage in a cell-mediated response. Interleukins secreted by the helper T cells stimulate the production of B cells.

Humans have learned to supplement natural body defenses. Three important approaches follow:

1. Antibiotics are chemicals derived from bacteria or fungi that are harmful to other microorganisms.

2. Vaccines are substances that stimulate the production of memory cells. Inactivated viruses or fragments of viruses, bacteria, or other microorganisms are used as vaccines. Once memory cells are formed, the introduction of the live microorganism will stimulate a swift response by the immune system before any disease can become established.

3. Passive immunity is obtained by transferring antibodies from an individual who previously had a disease to a newly infected individual. Newborn infants are protected by passive immunity through the transfer of antibodies across the placenta and by antibodies in breast milk.