Complement C4 Products

C4 consists of three polypeptide chains (alpha, molecular weight 95 kDa, beta, molecular weight 75 kDa and gamma, molecular weight 33 kDa) and is usually present in serum at 200-600mg/L. It is a protein of the classical complement pathway. When activated by C1s, a peptide C4a is released and the generated C4b binds C2 which is in turn cleaved by C1s to produce the classical pathway C3 convertase, namely C4b2a. C4b also has opsonin activity and binds to macrophages and neutrophils via their CR1 receptor.