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: 1  C5a plays an important role in chemotaxis.2  C5b forms the first part of the complement membrane attack complex.omplement component 5 is the fifth component of complement, which plays an important role in inflammatory and cell killing processes.