CFD, also known as adipsin, is involved in the alternative complement pathway of the complement system where it cleaves factor B. This protein is a member of the trypsin family of peptidases. It is a component of the alternative complement pathway best known for its role in humoral suppression of infectious agents. It is also a serine protease that is secreted by adipocytes into the bloodstream. Finally, it has a high level of expression in fat, suggesting a role for adipose tissue in immune system biology. Recombinant human CFD protein, fused to His-tag at N-terminus, was expressed in E.coli.