Complement C4, as a key component of the classical complement pathway, plays an important role in innate immune function. The C4 protein is expressed in the form of a single chain precursor, which is cleaved by protein hydrolysis into a trimer composed of alpha, beta, and gamma chains before secretion. This trimer provides a binding surface for the interaction between antigen antibody complexes and other complement components. The hydrolysis and degradation of complement C4 protein produce C4a allergen toxin, which is a mediator of local inflammatory response. It can induce smooth muscle contraction, increase vascular permeability, and promote histamine release from mast cells and eosinophils.
