Factor H, a 155 kD soluble glycoprotein, is a member of the regulators of complement activation family and is a complement control protein. It circulates in human plasma at a concentration of 500 - 800 mg/ml that provides critical negative regulation to the alternative pathway of complement cascade. Factor H is secreted by Kupffer cells, hepatocytes, vascular endothelial cells, and platelets. Overactive factor H may result in reduced complement activity on pathogenic cells, increasing susceptibility to microbial infections. Underactive factor H may result in increased complement activity on healthy host cells, resulting in autoimmune diseases.