White blood cells (leukocytes). Lymphocytes

Lymphocytes constitute about 28–42 percent of the white cells of the blood, and they are part of the immune response to foreign substances in the body. Most lymphocytes are small, only slightly larger than erythrocytes, with a nucleus that occupies most of the cell. Some are larger and have more abundant cytoplasm that contains a few granules. Lymphocytes are sluggishly motile, and their paths of migration outside of the bloodstream are different from those of granulocytes and monocytes. Lymphocytes are found in large numbers in the lymph nodes, spleen, thymus, tonsils, and lymphoid tissue of the gastrointestinal tract. They enter the circulation through lymphatic channels that drain principally into the thoracic lymph duct, which has a connection with the venous system. Unlike other blood cells, some lymphocytes may leave and reenter the circulation, surviving for about a year or more. The principal paths of recirculating lymphocytes are through the spleen or lymph nodes. Lymphocytes freely leave the blood to enter lymphoid tissue, passing barriers that prevent the passage of other blood cells. When stimulated by antigen and certain other agents, somelymphocytes are activated and become capable of cell division.

The lymphocytes regulate or participate in the acquired immunity to foreign cells and antigens. They are responsible for immunologic reactions to invading organisms, foreign cells such as those of a transplanted organ and foreign proteins and other antigens not necessarily derived from living cells. The two classes of lymphocytes are not distinguished by the usual microscopic examination but rather by the type of immune response they elicit. The B lymphocytes (or B cells) are involved in what is called humoral immunity. Upon encountering a foreign substance (or antigen), the B lymphocyte differentiates into a plasma cell, which secretes immunoglobulin (antibodies). The second class of lymphocytes, the T lymphocytes (or T cells), are involved in regulating the antibody-forming function of B lymphocytes as well as in directly attacking foreign antigens. T lymphocytes participate in what is called the cell-mediated immune response. T lymphocytes also participate in the rejection of transplanted tissues and in certain types of allergic reactions.

All lymphocytes begin their development in the bone marrow. The B lymphocytes mature partly in the bone marrow until they are released into the circulation. Further differentiation of B lymphocytes occurs in lymphoid tissues (spleen or lymph nodes), most notably on stimulation by a foreign antigen. In humans the precursors of the T lymphocytes migrate from the marrow to the thymus, where they differentiate under the influence of a hormonelikesubstance. (The thymus is a small organ lying just behind the breastbone in the upper portion of the chest. It is relatively large at birth, begins to regress after puberty, and may be represented only by a fibrous cord in the elderly. The thymus begins to exert its effects on thedifferentiation of lymphocytes before birth. The removal of the thymus from certain animals at birth prevents the normal development of immunologic responses.) Once they have matured, the T lymphocytes leave the thymus and circulate through the blood to the lymph nodes and the spleen. The two classes of lymphocytes originally derived their names from investigations in birds, in which it was found that differentiation of one class of lymphocyte was influenced by the bursa of Fabricius (an outpouching of the gastrointestinal tract) and thus was called the B lymphocytes, and the other was influenced by the thymus and was called the T lymphocytes.

A primary function of lymphocytes is to protect the body from foreign microbes. This essential task is carried out by both T lymphocytes and B lymphocytes, which often act in concert. The T lymphocytes can only recognize and respond to antigens that appear on cell membranes in association with other molecules termed major histocompatibility complex antigens. The latter are glycoproteins that present the antigen in a form that can be recognized by T lymphocytes. In effect, T lymphocytes are responsible for continuous surveillance of cell surfaces for the presence of foreign antigens. By contrast, the antibodies produced by B lymphocytes are not confined to recognizing antigens on cell membranes; they can bind to soluble antigens in the blood or extravascular fluids. T lymphocytes typically recognize antigens of infectious organisms that must penetrate cells in order to multiply, such as viruses. During their intracellular life cycle, viruses produce antigens that appear on the cell membrane. Two classes of T lymphocytes can be involved in the response to those cell-associated viral antigens: cytotoxic T lymphocytes, which destroy the cells by a lytic mechanism; and helper T lymphocytes, which assist B cells to produce antibodies against the microbial antigens. Helper T lymphocytes exert their influence on B lymphocytes through several hormone-like peptides termed interleukins (IL). Five different T lymphocyte interleukins (IL-2, IL-3, IL-4, IL-5, and IL-6) have been discovered, each with different (and sometimes overlapping) effects on B lymphocytes and other blood cells. Interleukin-1, produced by macrophages, is a peptide that stimulates T lymphocytes and that also acts on the hypothalamus in the brain to produce fever. The ability to develop an immune response (i.e., the T cell-mediated and humoral immune responses) to foreign substances is called immunologic competence (immunocompetence). Immunologic competence, which begins to develop during embryonic life, is incomplete at the time of birth but is fully established soon after birth. If an antigen is introduced into the body before immunologic competence has been established, an immune response will not result upon reinfection, and that person is said to be tolerant to that antigen.

Study of immunologic competence and immune tolerance has been accelerated by interest inorgan transplantation. The success rates of organ transplantations have been improved by better knowledge about donor selection and improved techniques for suppressing the immune responses of the recipient. An important element in donor selection is tissue typing: the matching of the donor's histocompatibility antigens (human leukocyte antigens) with those of the prospective recipient. The closer the match, the greater the probability that the graft will be accepted. (See also transplant.)