C4b2a

C4b2a is a molecular complex that plays a crucial role in the complement system, a part of the innate immune system. Specifically, it is the central enzyme of the classical and lectin pathways of complement activation. This complex is formed when the larger fragment of complement protein C4, known as C4b, binds to the larger fragment of complement protein C2, known as C2b. However, the nomenclature can be confusing as it is the C2b fragment of C2 that binds to C4b, and the resulting complex is historically referred to as C4b2a.

The formation of C4b2a is a key step in amplifying the complement cascade. Once assembled, C4b2a acts as a protease, meaning it can cleave other proteins. Its primary targets are complement protein C3 and complement protein C5. By cleaving C3, it generates C3a and C3b fragments. C3b can then initiate further complement amplification or opsonization, tagging pathogens for destruction. The cleavage of C5 by C4b2a generates C5a and C5b fragments, initiating the formation of the membrane attack complex (MAC), which can lyse target cells. Therefore, C4b2a is essential for effective immune responses against pathogens and damaged cells. Its dysregulation can lead to various autoimmune and inflammatory diseases.