C4A and autoimmune disease: While the complement activation of the downstream of complement C3 resulting in inflammatory molecules, such as C3a, C5a, and the membrane attack complex (MAC), plays a less important role, the early components of the classical pathway, such as C1q, C4, and C2, are more critical in maintaining homeostasis and lack of some of early components of classical pathway will predispose an individual to autoimmune disorders.