Figure 84

Human leukocyte antigens (HLAs) are heterodimeric cell-surface glycoproteins. HLAs are divided into two classes, according to their biochemical structure and respective functions. Class I antigens (A, B, and C) have a molecular weight of approximately 56,000 D and consist of two chains: a glycoprotein heavy chain (a) and a light chain (^-microglobulin). The a chain is attached to the cell membrane, whereas ^-microglobulin is associated with the a chain but is not covalently bonded. The HLA class I molecules are found on almost all cells; however, only vestigial amounts remain on mature erythrocytes. Class II antigens (HLA-DR, DQ, and DP) have a molecular weight of approximately 63,000 D and consist of two dissimilar glycoprotein chains, designated a and p, both of which are attached to the membrane. Each chain consists of two extramembranous amino acid domains, and the outer domains of each molecule contain the variable regions corresponding to class II alleles. Although class I antigens are expressed on all nucleated cells of the body, the expression of class II antigens is more restricted. Class II antigens are found on B lymphocytes, activated T lymphocytes, monocyte-macrophages, dendritic cells, and early hematopoietic cells, and of importance in transplantation, endothelial cells.

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