1. Cell membrane molecules that are used to classify leukocytes into subsets. CD molecules are defined or classified by the reference monoclonal antibodies to which they bind. There are many clusters of differentiation; many cells express more than one CD marker. A phenotypic characterization of a cell of hematopoietic origin may be made by examining the pattern of CD markers expressed at any given time. The physiologic function has been identified for many, but not all, known CDs. There are four general types: type I transmembrane proteins have their COOH-termini in the cytoplasm and their NH2-termini outside the cell; type II transmembrane proteins have their NH2-termini in the cytoplasm and their COOH-termini outside the cell; type III transmembrane proteins cross the plasma membrane more than once and hence may form transmembrane channels; and glycosylphosphatidylinositol-anchored proteins (type IV), which are tethered to the lipid bilayer through a glycosylphosphatidylinositol anchor.
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