What is cleaved as a result of activation of the classical complement pathway?

The activation of the classical complement pathway is characterized by a series of sequential proteolytic cleavages of complement proteins, resulting in the formation of various components that are crucial for the immune response. In this pathway, the first significant change is the cleavage of C4.

Upon the binding of an antibody to an antigen, the C1 complex is activated, leading to the cleavage of C4 into C4a and C4b. C4b then combines with C2, which is also cleaved by the C1 complex, forming the C3 convertase (C4b2a). This C3 convertase is essential for the subsequent cleavage of C3, which is a pivotal step in the complement cascade, leading to opsonization of pathogens, recruitment of inflammatory cells, and formation of the membrane attack complex.

While C3, C5, and C2 are all important components in the complement system and play crucial roles in the immune response, the specific focus of the classical complement pathway activation is on the cleavage of C4.