The complement system is a crucial component of the innate immunity against microbial infection. Complement factor H, a 155 kDaplasma glycoprotein, is an essential regulatory protein that plays a critical role in the homeostasis of the complement system in plasma and in the protection of bystander host cells and tissues from damage by complement activation. Factor H binds to C3b, accelerates the decay of the alternative pathway C3-convertase and acts as a cofactor for the factor I-mediated proteolyticinactivation of C3b. In addition, factor H has multiple physiological activities 1) acts as an extracellularmatrix component, 2) binds to cellular receptors of the integrintype, and 3) interacts with a wide selection of ligands, such as the C-reactive protein, thrombospondin, bone sialoprotein, osteopontin, and heparin. Complement factor H has revealed an association with two different renal diseases, glomerulonephritisand atypical hemolytic uremicsyndrome (aHUS). Factor H functions as a cofactor in the inactivation of C3b by factor I and also increases the rate of dissociation of the C3bBb complex (C3 convertase) and the (C3b)NBB complex (C5 convertase) in the alternative complement pathway.