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The leukocyte-common antigen (L-CA) family is a group of high molecular weight glycoproteins uniquely expressed on the surface of all leukocytes and their hemopoietic progenitors ( 114). Members of this family differ by both protein sequence and carbohydrate structures and are expressed by leukocyte populations in specific patterns ( 15-27). An example of the differential expression is shown for the rat L-CA family in Figure 1 ( 19). Thymocytes express the lowest apparent molecular weight form of 1 80 kd; B lymphocytes express the highest form, 220 kd; and T lymphocytes express multiple forms. Differences also exist between T-cell subsets (Figure 1 C); CD8 T cells (Te/s) express the higher molecular weight forms more abundantly than do CD4 T cells (T H) ' The cell-type-specific patterns of expression are conserved throughout mammalian evolution ( 1, 2, 91 1, 19, 28), and there appear to be similar patterns of expression in chicken lymphocytes (29). L-CA is referred to in the literature by different names, including T200 (30), B220 for the B cell form ( 1 2) , the mouse allotypic marker Ly-5 ( 3 1 ) and more recently CD45 (32). L-CA is the most accurate descriptive name and is used for the purpose of this review. The L-CA family is a major cell surface component of lymphocytes and carries much of the carbohydrate of these cells. It has been estimated that 10% of the lymphocyte surface is occupied by one or more L-CA members (33). Because of this abundance, L-CA was easily detected on SDS-poly acrylamide gels of lymphocyte membranes (36-39) (Figure lA). It was also initially characterized as the major specificity of antilymphocyte sera (34) and as an allotypic marker (31 , 35). The primary protein structure has been determined from the analysis of cDNA clones, and this information,
M L Thomas (Sat,) studied this question.