Figure 911

Electron micrograph of a human basophil.

The nucleus appears as three separate bodies; the connecting strands are not in the plane of section. The basophil granules (B) are very large and irregularly shaped. Some granules reveal myelin figures (MF). M, mitochondria. x26,000. (Courtesy of Dr. Dorothea Zucker-Franklin.) Inset. Light microscopic appearance of a basophil from a blood smear. xl,800.

In the medium lymphocyte, the cytoplasm is more abundant, the nucleus is larger and less heterochromatic, and the Golgi apparatus is somewhat more developed (Fig. 9.12). Greater numbers of mitochondria and polysomes and small profiles of rough endoplasmic reticulum are also seen in these medium-sized cells. The ribosomes are the basis for the slight basophilia displayed by lymphocytes in stained blood smears.

Three functionally distinct types of lymphocytes are present in the body: T lymphocytes, B lymphocytes, and NK cells

The characterization of lymphocyte types is based on their function, not on their size or morphology. T lymphocytes (T cells) are so named because they undergo differentiation in the thymus. B lymphocytes (B cells) are so named because they were first recognized as a separate population in the bursa of Fabricius in birds or bursa-equivalent organs (e.g., bone marrow) in mammals. NK cells develop from the same precursor cell as B and T cells and are so named because they are programmed to kill certain types of transformed cells.

• T cells have a long life span and are involved in cell-me-diated immunity. They express CD2, CD3, and CD7 marker proteins on their surface; however, they are sub-classified on the basis of the presence or absence of CD4 and CD8 proteins. CD4+ T lymphocytes possess the CD4 marker and recognize antigens bound to major his-tocompatability complex II (MHC II) molecules. CD8 + T lymphocytes possess the CDS marker and recognize antigen bound to MHC I molecules.

• B cells have variable life spans and are involved in the production of circulating antibodies. Mature B cells in blood express IgM and IgD on their surface as well as MHC II molecules. Their specific markers are CD9, CD 19, CD20, and CD24.

• NK cells are programmed during their development to kill certain virus-infected cells and some types of tumor cells. NK cells are larger than B and T cells (-15 ¡xm in diameter) and have a kidney-shaped nucleus. Because NK cells have several large cytoplasmic granules easily seen by light microscopy, they are also called large granular lymphocytes (LGLs). Their specific markers include CD 16, CD56, and CD94.

T and B cells are indistinguishable in blood smears and tissue sections; immunocytochemical staining for different types of markers and receptors on their cell surface must be used to identify them (see below). NI< lymphocytes can be identified in the light microscope by size, nuclear shape, and presence of cytoplasmic granules; however, immunocytochemical staining for their specific markers is used to confirm microscopic identification.

T and B lymphocytes express different surface molecules

Although the T and B cells cannot be distinguished on the basis of their morphology, their distinctive surface proteins (CD proteins) can be used to identify the cells with immunolabeling techniques. In addition, immunoglobulin molecules (antibodies) are expressed on the surface of B cells that function as antigen receptors. In contrast, T cells do not have antibodies but express unique cell surface recognition proteins called T cell receptors (TCRs). These recognition proteins appear during discrete stages in the maturation of the cells within the thymus. In general, the surface molecules mediate or aug ment specific T cell functions and are required for the recognition or binding of T cells to antigens displayed on the surface of target cells.

In human blood, approximately 60 to 80% of the lymphocytes are mature T cells, and 20 to 30% are mature B cells. Approximately 5 to 10% of the cells do not demonstrate the surface markers associated with either T or B cells. These are NI< cells and the rare circulating hemopoietic stem cells (see below). The size differences described above may have functional significance; some of the large lymphocytes may be cells that have been stimulated to divide, whereas others may be plasma cell precursors that are undergoing differentiation in response to the presence of antigen.

Three fundamentally different types of T lymphocytes have been identified: cytotoxic, helper, and suppressor

The activities of cytotoxic, helper, and suppressor T lymphocytes are mediated by molecules located on their surface. Immunolabeling techniques have made it possible to identify specific types of T cells and study their function.

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