Polyclonal antibody HP9058 recognizes human complement factor H related protein 4 (FHR4). The complement system is the first line of defense against pathogens and facilitates elimination of apoptotic and damaged cells. Complement factor H (fH) is an abundant plasma protein that negatively regulates the alternative pathway and the C3b amplification loop. The proteins encoded by the fH gene family includes besides fH also five fH-related proteins (FHR1-5), encoded by closely linked but unique genes. Like fH these proteins consist of repeating short consensus repeats (SCRs). fH competes with fB for binding to C3b and C3b(H2O) thereby blocking the formation of the convertase, accelerates decay of C3-convertases and is a cofactor for fI. The function of FHR proteins is less clear. FHRs are produced primarily in the liver and secreted in the plasma. FHR4 binds to the C3d region of C3b. In contrast to fH, the FHR4-C3b interaction promotes complement activation. Two transcripts of the FHR4 gene exist: FHR4B and FHR4B, consisting of respectively 9 and 5 SCRs (86& 42kDa). Both variants have different binding profiles to C3b. Suggested ligand in complement activation of FHR4 is pentameric CRP.