These proteins help mediate the cell–cell interactions necessary for activities such as chemotaxis and cytotoxic killing.
Each of these proteins associates with a molecule of CD18 to form an α-β heterodimer that is then transported and expressed on the cell surface. LFA-1, CR3, and CR4 are routinely referred to as CD11a, CD11b, and CD11c respectively. CR3 is a member of the β2-integrin family of cell adhesion proteins that includes leukocyte functional antigen-1 (LFA-1) and CR4. Also, on activated monocytes, CR3 has been shown to bind fibrinogen and factor Xa (of the coagulation cascade). CR3 represents another complement receptor that binds to iC3b and β-glucan structures found on zymosan (yeast cell wall). CR2 is expressed on antigen-presenting cells, such as follicular dendritic cells and B cells, where it facilitates the process of antigen-immune complex-driven B cell proliferation, providing a link between innate and adaptive immunity. CR2 is structurally similar to CR1 (with 16 SCR domains see Figure II.2.4.7), but recognizes the C3d fragment of C3b that is bound to antigen. As discussed above, CR1 is also a complement regulatory protein. On RBCs, CR1 acts to transport C3b-immune complexes to the liver for metabolism. On neutrophils and monocytes, activated CR1 will facilitate the phagocytosis of C3b- and C4b-coated particles. CR1 is found on a variety of cells including RBCs, neutrophils, monocytes, B cells, and some T cells, and recognizes a site within the C3c region of C3b ( Figure II.2.4.2). These receptors are called Complement Receptor 1, 2, 3 or 4 (CR1, CR2, etc). The ability of complement to function in the opsonization of foreign elements is accomplished in large part by a set of receptors that recognize various C3 and C4 fragments bound to these foreign surfaces. PMN, monocytes, B cells/respiratory burst activityī cells, monocytes/secretion of factor I, respiratory burst activity Mast cells, eosinophils (various tissues)/histamine release, IL-6 production PMN, monocytes, Mast cells, T cells, epithelial and endothelial cells, hepatocytes, CNS, fibroblasts/chemotaxis, degranulation, hyperadherence, respiratory burst, cytokine production (IL-6,IL-8) PMN, monocyte/phagocytosis of microorganisms, respiratory burst activity PMN, platelets, B cells/leukocyte-endothelial cell interactionĬR3/ iC3b, ICAM-1, βglucan zfibrinogen, factor Xa RBC, PMN, monocytes, B, and T cells/clearance of immune complexes, phagocytosis, facilitates cleavage of C3b to C3dg by Factor I