Immunity And The Lymphatic System

The immune system consists of defenses against foreign matter that gains entry to the body. It consists of the lymphatic system plus components of numerous other systems of the body. Many toxic pollutants either stimulate or suppress the immune system.

The lymphatic system consists of lymph, lymphatic vessels, lymphocytes, and lymphoid tissues and organs. Lymph is a fluid similar to plasma. Lymphatic vessels are similar in structure to veins. They conduct lymph from peripheral tissues to the veins. Capillaries deliver more liquid to tissues than they carry away. The rest forms intercellular fluid that collects as lymph. The most important lymphoid organs are the lymph nodes, the spleen, and the thymus. The lymph nodes contain immune system cells that remove pathogens from the lymph before they reach the bloodstream. The tonsils are lymph nodes positioned to respond to infections arriving by way of the mouth or nose. The thymus produces T cells (described below). The spleen performs functions for the blood similar to those the lymph nodes perform for the lymph, such as removal of cellular debris and responding to pathogens in the blood. It also stores iron from recycled erythro-cytes.

The defenses of the body are classified into two forms: specific and nonspecific. Nonspecific defenses protect without discriminating the exact type of threat, and act relatively rapidly. There are seven kinds of nonspecific defenses:

1. Barrier defenses include the skin, mucous membranes, and hair, which act to prevent physical access to the interior by disease agents.

2. Phagocytes are cells that engulf pathogens (the process of phagocytosis) and debris. Several of the white blood cells serve this function. In addition, the white blood cells known as monocytes are converted into phagocytes called macrophages, which perform this function in tissues and the lymphatic system.

3. Immunological surveillance involves a kind of lymphocyte called natural killer (NK) cells that can recognize and kill virus-infected and cancerous cells. They are also recruited by the specific defense immune system.

4. Interferons are small proteins released by lymphocytes and macrophages that stimulate virus-infected cells to produce antiviral proteins.

5. Complement consists of 11 special proteins in plasma that work either alone, forming pores in the membrane of foreign cells, killing them, or in conjunction with the specific defenses to have the same effect. They can also attack virus structure, attract phagocytes, and stimulate inflammation.

6. Inflammation results when damage stimulates the release of histamines, heparin, prostaglandins, potassium, and other substances which produce dilation and increased permeability of capillaries. These produce pain, swelling, warmth, and redness of the injured area. Clots isolate the area, and macrophages are attracted. The increased temperature can reduce pathogen growth.

7. Fever is the increase of body temperature above 37.2°C (99°F). Fever is caused by proteins in the blood called pyrogens, Some pyrogens are produced by macrophages, which affect the temperature set point. High body temperature may be a strategy to inhibit growth of pathogens while enhancing the body's own metabolism. Metabolism increases about 10% for each 1°C increase in temperature.

Specific defenses are highly selective: for example, recognizing a particular strain of bacteria but ignoring all others. However, they take longer to get into action. The protection of specific defenses is called immunity. Immune responses are stimulated by foreign substances called antigens. Specific defenses refer to immune responses that recognize particular antigens. Immunity is mediated by cells that differentiate from lymphocytes that have left the bloodstream. These can form three basic types: NK cells, mentioned above; B cells; and T cells. Both B and T cells are produced in the lymphoid organs.

There are several basic forms of T cells. The most important are the cytotoxic T cells (TC cells, also called killer T cells). TC cells recognize antigens bound in the membranes of other cells. These other cells can be any cell in the body. All our cells constantly take protein fragments from the cytoplasm and place them on their membranes complexed with a glycoprotein called the major histocompatibility complex (MHC). If a cell is damaged or contains infectious material such as proteins, some of the antigens presented on the surface will be abnormal. The deranged cell will also produce other membrane proteins that signal, in effect, "I am deranged.'' A TC cell can bind to the MHC and the second signal protein. The presence of both binding sites stimulates the TC cell to kill the deranged cell either by producing toxins or by stimulating the cell to kill itself (a process called apoptosis). The now-activated TC cell can then reproduce many times, generating an army of cells to attack other cells carrying the same antigen. The reproduction process will also generate memory TC cells, which remain in reserve in case of future reinfections by the same antigen-bearing pathogen.

B cells act to eliminate the source of antigens directly instead of killing infected cells. The lymph nodes store millions of different populations of B cells, each capable of reacting to a different antigen. Almost any biological substance, whether from a fungus or a transplanted organ, can find a B cell that can react to it. Each population of B cells has a specific type of globulin protein studding its surface. These are Y-shaped molecules called antibodies, or immunoglobulins (Figure 9.9). Antibodies are produced by B cells. Once an antigen complexes with the antibodies on the surface of a B cell, the B cell becomes sensitized. It then reacts with a helper T cell and becomes activated. The activated B cell then starts reproducing rapidly. Some of the offspring become plasma cells, which produce antibodies that circulate in the blood. Others become memory B cells, which remain in storage in case of a repeat infection.

The attack is now carried on by the circulating antibodies. The top of the Y can complex with antigens. When macrophages encounter a substance covered with antibodies, it is stimulated to phagocytize it, destroying it. Complement also reacts with the antibodies, and if they are attached to a cell or a virus, the complement will destroy them.

Several varieties of polysaccharides on the surface of red blood cells differ between persons to give them their characteristic blood type. These sugars are natural antigens that act when blood from one person is given to another in a transfusion. The genetic

Binding sites

Heav chaii

Binding sites

Heav chaii

Light chain

Light chain

Figure 9.9 Basic structure of an immunoglobulin protein. (Based on Van de Graaff and Rhees, 1997.)

TABLE 9.4 Human ABO Blood Types and Their Compatibilities

Blood

Percent of U.S.

Antigen on

Genetic

Antibodies

Compatible Blood

Type

Population

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