Complement factor D, also known as Adipsin, C3 convertase activator, Properdin factor D and CFD is a secreted protein which belongs to the peptidase S1 family. CFD / Adipsin contains one peptidase S1 domain. Complement factor D ( CFD / Adipsin ) is a component of the alternative complement pathway best known for its role in humoral suppression of infectious agents. Complement factor D ( CFD / Adipsin ) has a high level of expression in fat, suggesting a role for adipose tissue in immune system biology. This protein is also a serine protease that is secreted by adipocytes into the bloodstream. Complement factor D ( CFD / Adipsin ) cleaves factor B when the latter is complexed with factor C3b, activating the C3bbb complex, which then becomes the C3 convertase of the alternate pathway. Its function is homologous to that of C1s in the classical pathway. Complement factor D ( CFD / Adipsin ) is a serine protease that stimulates glucose transport for triglyceride accumulation in fats cells and inhibits lipolysis. Defects in CFD / Adipsin are the cause of complement factor D deficiency (CFD deficiency) which predisposes to invasive meningococcal disease.