Complement component 5 is a protein that in humans is encoded by the C5 gene. Complement component 5 is involved in the complement system. It is cleaved into C5a and C5b: C5a plays an important role in chemotaxis; C5b forms the first part of the complement membrane attack complex. Deficiency is thought to cause Leiner's disease. Complement component 5 is the fifth component of complement, which plays an important role in inflammatory and cell killing processes. This protein is composed of alpha and beta polypeptide chains that are linked by a disulfide bridge. An activation peptide, C5a, which is an anaphylatoxin that possesses potent spasmogenic and chemotactic activity, is derived from the alpha polypeptide via cleavage with a C5-convertase. The C5b macromolecular cleavage product can form a complex with the C6 complement component, and this complex is the basis for formation of the membrane attack complex, which includes additional complement components.