Box 131

Functional Considerations: Origin of the Names T Lymphocyte and B Lymphocyte

In the early 1960s, investigators using chicken embryos demonstrated that the bursa of Fabricius, a mass of lymphatic tissue associated with the cloaca of birds, was one of the anatomic sites of lymphocyte differentiation. When this tissue was destroyed in the chicken embryos (by either surgical removal or administration of high doses of testosterone), the adult chickens were unable to produce antibodies, leading to impaired humoral immunity. The chickens also demonstrated a marked reduction in the number of lymphocytes found in specific bursa-dependent areas of the spleen and lymph nodes. These affected lymphocytes were therefore named B lymphocytes or B cells. The bursa-equivalent organs in mammals (including humans) are the GALT and the bone marrow, where B lymphocytes differentiate into immunocompetent cells. Thus, the "B" refers to the bursa of birds or the "bursa-equivalent organs" of mammals.

Investigators studying newborn mice found that removal of the thymus results in profound deficiencies in cell-mediated immune responses. The rejection of transplanted skin from a heterologous donor is an example of cell-mediated immune response. Thymectomized mice demonstrate a marked reduction in the number of lymphocytes found in specific regions of the spleen and the lymph nodes (thymus-dependent areas). The areas of depletion differ from those identified after removal of the bursa of Fabricius in the chicken. These affected lymphocytes were therefore named T lymphocytes or T cells; the "T" refers to the thymus.

nonspecific defense known as the inflammatory response. The inflammatory response may either sequester the antigen, physically digest the antigen with enzymes secreted by neutrophils, or phagocytose and degrade the antigen in the cytoplasm of macrophages. Degradation of antigens by macrophages may lead to subsequent presentation of a portion of the antigen to immunocompetent lymphocytes to elicit a specific immune response.

Specific immune responses are either primary or secondary

When immunocompetent cells encounter a foreign antigen (e.g., antigen associated with pathogenic microorganisms, tissue transplants, or toxins), a specific immune response to the antigen is generated.

A primary immune response refers to the body's first encounter with an antigen. This response is characterized by a lag period of several days before antibodies (mostly IgM) or specific lymphocytes directed against the invading antigen can be detected in the blood. The initial response to an antigen is initiated by only one or a few B lymphocytes that have been genetically programmed to respond to that specific antigen. Following this initial immune response, a few antigen-specific B lymphocytes remain in circulation as memory cells.

The secondary immune response is usually more rapid and more intense (characterized by higher levels of secreted antibodies, usually of the IgG class) than the primary response, because of the presence of specific memory B lymphocytes already programmed to respond to that specific antigen. The secondary response is the basis of most immunizations for common bacterial and viral diseases. Some antigens, such as penicillin and insect venoms, may trigger an intense secondary immune response that produces a hypersensitivity reaction or even anaphylaxis (see page 365). However, antibodies themselves do not kill or destroy invading antigens; they simply mark them for destruction by cells of the immune system.

The two types of specific immune responses are the humoral and cell-mediated responses

In general, an encounter with a given antigen triggers a response characterized as either a humoral immune response (antibody production) or a cell-mediated immune response. Typically, however, both cellular and humoral immune systems are involved, although one system generally predominates, depending on the stimulus.

• Humoral immunity is mediated by antibodies that act directly on an invading agent. These antibodies are produced by B lymphocytes and by plasma cells derived from B lymphocytes. In some diseases, e.g., tetanus, a nonimmune person can be rendered immune by receiving an injection of antibody purified from the blood of an immune person or animal. The effectiveness of this passive transfer proves that it is the antibody that is responsible for the protection.

• Cell-mediated immunity is mediated by specific T lymphocytes that attack and destroy virus-infected host cells or foreign cells. Cell-mediated immunity is important in the defense against viral, fungal, and mycobacterial infections, as well as tumor cells. Cell-mediated immunity is also responsible for transplant rejection.

Helper T and cytotoxic lymphocytes recognize and bind to antigens that are bound to MHC molecules

To understand how the specific immune responses (humoral and cell-mediated responses) are initiated, one must grasp the central role played by the helper and cytotoxic T lymphocytes. Helper T and cytotoxic lymphocytes act as the immune system "patrols." Both kinds of lymphocytes have a T cell receptor (TCR), a transmembrane protein whose exposed portion is on the T cell membrane in close proximity to the CD3 marker (Fig. 13.3). The TCR recognizes antigen only when it is attached to "identification molecules," the MHC molecules. In addition, helper T

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