Glycoprotein that plays an essential role in maintaining a well-balanced immune response by modulating complement activation. Acts as a soluble inhibitor of complement, where its binding to self markers such as glycan structures prevents complement activation and amplification on cell surfaces (PubMed:21285368, PubMed:21317894, PubMed:25402769). Accelerates the decay of the complement alternative pathway (AP) C3 convertase C3bBb, thus preventing local formation of more C3b, the central player of the complement amplification loop (PubMed:19503104, PubMed:21317894, PubMed:26700768). As a cofactor of the serine protease factor I, CFH also regulates proteolytic degradation of already-deposited C3b (PubMed:18252712, PubMed:23332154, PubMed:28671664). In addition, mediates several cellular responses through interaction with specific receptors. For example, interacts with CR3/ITGAM receptor and thereby mediates the adhesion of human neutrophils to different pathogens. In turn, these pathogens are phagocytosed and destroyed (PubMed:20008295, PubMed:9558116). {ECO:0000269|PubMed:18252712, ECO:0000269|PubMed:19503104, ECO:0000269|PubMed:20008295, ECO:0000269|PubMed:21285368, ECO:0000269|PubMed:21317894, ECO:0000269|PubMed:23332154, ECO:0000269|PubMed:25402769, ECO:0000269|PubMed:26700768, ECO:0000269|PubMed:28671664, ECO:0000269|PubMed:9558116}. (Microbial infection) In the mosquito midgut, binds to the surface of parasite P.falciparum gametocytes and protects the parasite from alternative complement pathway-mediated elimination. . This is the function of Ensembl gene identifier ENSG00000000971 (CFH, complement factor H).