– Complement factor D, also known as adipsin, is a member of the chymotrypsin family of serine proteases, which plays an essential role in host defense as the rate-limiting enzyme in the alternative pathway of complement activation. Complement factor D activates a convertase (C3bBb) responsible for cleavage of the complement protein C3, which leads to the activation of terminal complement component C5-9 to form the membrane attack complex on microbial or cellular surfaces. It also functions in the regulation of systemic energy balance and physiologic and pathologic processes, including immunity and inflammation.