C4b2a is a protein complex that plays a crucial role in the immune system, specifically in the complement pathway. It is formed when the protein C4b binds to C2, leading to the activation of the complement system, which helps the body fight infections and clear damaged cells. This complex is part of the classical pathway of complement activation, which is triggered by antibodies bound to pathogens. The formation of C4b2a enhances the immune response by promoting inflammation and facilitating the opsonization of pathogens, making them easier for immune cells to recognize and eliminate.