Complement factor H (CFH) is the first regulatory protein of the alternative pathway of the complement system. There are three pathways of complement activation. The classical pathway is initiated by Immune complexes; the lectin pathway by surface bound mannan binding lectin; and the alternative pathway by all the surfaces that are not specifically protected against it. Each generates a C3 convertase, a serine protease that cleaves the central complement protein C3, and generates the major cleavage fragment C3b. The complement system mediates a number of essential biological functions that participate in host defense against infection, initiation of the inflammatory reaction, processing and clearance of immune complexes and regulation of the immune response.CFH is a single-chain serum glycoprotein of 150 kD with a modular structure consisting of a tandem of 20 homologous units of about 60 amino acid, called short consensus repeats (SCR).Numerous functional sites have been identified along the 20 SCR domain structure of factor H. Three C3-binding sites have been identified; in the SCR1-4 in SCR6-10 and SCR13-20. Three polyanion binding sites like heparin and several glycoaminoglycans have also been identified in the SCR7, 13 and 20. FH displays anti inflammatory functions and acts as a ligand for CRP.CFH has two important functional domains that are located at the opposite ends of the protein. The N-terminal fragment of the factor H molecule is an essential fluid phase regulator of the alternative pathway. With the C terminal domain and SCR 7 CFH binds to cell and tissue surface and thus mediates its protective role also on host cell surface. CFH is a relatively abundant plasma protein, with a concentration of 400-800 µg/ml, that is essential to maintain complement homeostasis and to restrict the action of complement to activating surfaces. CFH binds to C3b, accerates the decay of the alternative pathway C3-convertase (C3bBb) and act as co-factor for the factor I-mediated proteolytic inactivation of C3b. Factor H regulates complement both in fluid phase and on cellular surfaces.Genetic analyses reveal a clear association of CFH with different human diseases. These incude diseases of the kidney, the atypical form of Hemolytic Uremic Syndrome (aHUS) and membranoproliferative glomerulonephritis (MPGN), and of the eye, age-related macular degeneration (AMD)