Leukocytes remove invading antigens (for example, bacteria) and to some extent transport and distribute antibodies. Monocytes and all of the granulocytes have been shown to demonstrate directional movement. Their movement is subject to chemotaxis or the response of living protoplasm to a chemical stimulus. As mentioned previously, the attraction of the leukocytes to bacteria and certain other particles by substances released by the particles is called chemotaxis. They either transport the particles or engulf them. The chemotaxis process of engulfing and destroying bacteria, or phagocytosis, is a prime function of leukocytes.
Monocytes. These cells will engulf bacteria and larger materials, including even protozoa and red cells, and are transformed into and/or are called macrophages. In this regard, monocytes are perhaps the most efficient phagocytes of all the cells. Monocytes contain many of the lytic enzymes that are found in microphages (granulocytes). In addition, monocytes contain lipases that enable them to dissolve the lipoid capsules of certain bacteria.
Neutrophils. Neutrophilic leukocytes are excellent microphages. That is, they engulf bacteria and other microscopic particles. The particles are first surrounded by cellular pseudopodia and then incorporated into a cell vacuole. There the foreign bodies mix with substances released from the cytoplasm of leukocytes. In this way the leukocyte is not injured by whatever "combat activity" is taking place in the vacuole. Neutrophils are fully developed (mature) cells that are incapable of mitotic division. They carry on active metabolism. Eventually the granulocytes disintegrate and in inflammatory processes are succeeded by monocytes.
Eosinophils. The eosinophils are easily distinguishable by their large cytoplasmic granules. They have some similarities with the neutrophil: they arise from a common stem cell and share same morphologic features. Unlike the neutrophil, however, eosinophils initially develop in the bone marrow, then are released into the circulation, and undergo further maturation in the spleen. Their emergence from the marrow seems to depend upon a stimulus provided by lymphocytes. Eosinophils are found in tissue fluid as well as in peripheral blood, especially in areas where there is an allergic reaction. Current thinking holds that eosinophils are involved in antigen- antibody reactions, and have been shown to phagocytize antigen-antibody reactants. Eosinophils are also thought to transport, or at least contain, lysins that act on fibrin. It is suggested that eosinophils limit the action of substances such as histamine. How this is accomplished is not yet clear. The mobilization of eosinophils from their reserve in the bone marrow is at least in part under hormonal control. If the adrenal cortex is functioning properly, an injection of adrenocorticotropic hormone (ACTH) results in a marked decrease in the number of circulating eosinophils and in the number of circulating lymphocytes. On the other hand, there is an increase in the number of circulating neutrophils.Basophils. The function of basophils in man has not been ascertained.
They quite possibly represent a vestige of evolution. Their granules have been found to contain heparin and these cells frequently appear during the clot dissolution phase of an injury. Hence, it has been suggested they may be involved in clot absorption.
Lymphocytes. The lymphocyte is now believed to be directly connected with antibody production. Undoubtedly, the lymphocyte performs important immunologic functions. According to very recent studies, many of the activities previously thought to take place in the reticuloendothelial (RE) system actually take place in lymphocytic tissue.
Table 1-2. Blood cell functions.