CR3 Complement Receptor Mac1 CD17bCD1

CR3 is found on monocytes, neutrophils, NK cells, and dendritic cells. Structurally, CR3 has homologies with the integrin family of cell surface proteins. CR3 consists of two noncovalently linked a (165 kDa) and B (95 kDa) chains designated CD18. The CR3 B chain has homology with the B chains of lymphocyte function-associated antigen (LFA-1) and pl50, 95. Because of the similarity to LFA-1, monocytes and neutrophils can attach to the vascular epithelium via ICAM-1 without complement activation.

Phagocytosis and killing of xenobiotics are initiated by two distinct mechanisms. Like other complement receptors, CR3 binds iC3b liberated by Factor I

cleavage and initiates phagocytosis and intracellular killing of bacteria. CR3 binds lectins present on the surface of Saccharomyces, Staphylococcus epider-miditis, and Histoplasma capsulatum without the necessity for complement activation (Berger et al., 1981).

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