The complement receptors CR3 and CR4 are heterodimeric integrin-type receptors comprising of alpha-chains, CD11b for CR3 or CD11c for CR4, that are associated non-covalently with a common beta chain CD18. In this drawing (A) CR3 and (B) CR4 are shown in their inactive and bent forms. The schematic pictures of the primary protein domains in the alpha-chains of the β2-integrins show (C) the locations of CR3-related mutations (M441K, T1000N) and (D) the CR4-related mutations (W48R, A251T, E547K).