Factor D (EC 3.4.21.46, C3 proactivator convertase, properdin factor D esterase, factor D (complement), complement factor D, CFD, adipsin) is a protein which in humans is encoded by the CFD gene. Factor D is involved in the alternative complement pathway of the complement system where it cleaves factor B. The protein encoded by this gene is a member of the trypsin family of serine proteases secreted by adipocytes into the bloodstream. The encoded protein is a component of the alternative complement pathway best known for its role in humoral suppression of infectious agents. Finally, the encoded protein has a high level of expression in fat, suggesting a role for adipose tissue in immune system biology. Factor D is a serine protease that stimulates glucose transport for triglyceride accumulation in fats cells and inhibits lipolysis.