Figure 912

Electron micrograph of a medium-sized lymphocyte. The punctate appearance of the cytoplasm is due to the presence of numerous free ribosomes. Several mitochondria (M) are evident. The cell center or centrosphere region of the cell (the area of the nuclear indentation) also shows a small Golgi apparatus (G) and a centriole (C). x 26,000. (Courtesy of Dr. Dorothea Zucker-Franklin.) Inset. Light microscopic appearance of a medium-sized lymphocyte from a blood smear, x 1,800.

Cytotoxic CD8+ T lymphocytes serve as the primary effector cells in cell-mediated immunity. CD8+ cells are specifically sensitized T lymphocytes that recognize antigens through the TCRs on host cells that are infected with viruses or have become neoplastic. Cytotoxic CD8+ T lymphocytes only recognize antigens bound to MHC I molecules. After the TCR binds the antigen-MHC I complex, the cytotoxic CDS' T lymphocyte secretes lym-phokines and perforins that produce ion channels in the membrane of the infected or neoplastic cell, leading to its lysis (see Chapter 13). Cytotoxic CD8+ T lymphocytes play a significant role in rejection of allografts and in tumor immunology.

Helper CD4+ T lymphocytes are critical for induction of an immune response to a foreign antigen. Antigen bound to MHC II molecules is presented by antigen-presenting cells such as macrophages to a helper CD4+ T lymphocyte. Binding of the TCR to the antigen-MHC II complex activates the helper T lymphocyte. The activated helper CD4+ T lymphocyte then produces interleukins (mainly IL-2), which act in an autocrine mode to simulate the proliferation and differentiation of more helper CD4+ T lymphocytes. Newly differentiated cells synthesize and secrete lymphokines that affect function as well as differentiation of B cells, T cells, and NIC cells. B cells differentiate into plasma cells and synthesize antibody. Suppressor and/or cytotoxic CD8+, CD4SRA+, T lymphocytes diminish or suppress antibody formation by B cells. They also downregulate the ability of T lymphocytes to initiate a cellular immune response. The sup pressor and/or cytotoxic T cells may also function in the regulation of erythroid cell maturation in the bone marrow.

Monocytes

Monocytes are the precursors of the cells of the mononuclear phagocytotic system

Monocytes are the largest of the WBCs in a blood smear (average diameter, 18 pm). They travel from the bone marrow to the body tissues, where they differentiate into the various phagocytes of the mononuclear phagocytotic system, i.e., connective tissue macrophages (histiocytes), osteoclasts, alveolar macrophages, perisinusoidal macrophages in the liver (Kupffer cells), and macrophages of lymph nodes, spleen, and bone marrow, among others (see Chapter 5). Monocytes remain in the blood for only about 3 days.

The nucleus of the monocyte is typically more indented than that of the lymphocyte (Fig. 9.13). The indentation is the site of the cell center where a well-developed Golgi apparatus and centrioles are located. Monocytes also contain smooth endoplasmic reticulum, rough endoplasmic reticulum, and small mitochondria. Although these cells are classified as agranular, they contain small, dense, azurophilic granules. These granules contain typical lysosomal enzymes similar to those found in the azurophilic granules of neutrophils.

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