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. Natural killer (NK) cells
FIGURE 10.11 • Electron micrograph of a medium-sized lymphocyte. The punctate appearance of the cytoplasm is caused by 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). 26,000. (Courtesy of Dr. Dorothea Zucker-Franklin.) Inset. Light microscopic appearance of a medium-sized lymphocyte from a blood smear. 1,800.
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-mediated immunity. T cells are characterized by the presence of cell-surface recognition proteins called T-cell receptors (TCRs), which in a majority of T cells comprise two glycoprotein chains called and  -TCR chains. They express CD2, CD3, CD5, and CD7 marker proteins on their surface; however, they are subclassified 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 histocompatability complex II (MHC II) molecules. CD8 T lymphocytes possess the CD8 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 and MHC II molecules on their surface. Their specific markers are CD9, CD19, CD20, and CD24.
 NK cells are programmed during their development to kill certain virus-infected cells and some types of tumor cells. They also secrete an antiviral agent, interferon  (IFN-). NK cells are larger than B and T cells (15 m 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 granu lar lymphocytes (LGLs). Their specific markers include CD16, 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. NK 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, immunoglobulins are expressed on the surface of B cells that function as antigen receptors. In contrast, T cells do not have antibodies but express TCRs. These recognition proteins appear during discrete stages in the maturation of the cells within the thymus. In general, the surface molecules mediate or augment specific T-cell functions and are required for the recognition or binding of T cells to antigens displayed on the surface of target cells.