Abstract

The classical complement pathway is a crucial component of the immune system, activated primarily when antibodies bind to the surface of pathogens such as bacteria. This binding event marks the initiation of a cascade of proteolytic reactions that ultimately lead to the destruction of the pathogen. The process begins with the recognition and attachment of antibodies, specifically IgG or IgM, to bacterial antigens. This antibody-antigen complex then interacts with the C1 complex, consisting of C1q, C1r, and C1s subcomponents. The binding of C1q to the Fc region of the antibody triggers the activation of C1r and C1s, which subsequently cleave and activate the next components in the pathway, C4 and C2. The cleavage products, C4b and C2a, form the C3 convertase (C4b2a complex), which plays a pivotal role in amplifying the response by cleaving C3 into C3a and C3b. C3b acts as an opsonin, marking the pathogen for phagocytosis, while C3a functions as an anaphylatoxin, promoting inflammation. This classical pathway, therefore, is essential for enhancing the ability of the immune system to recognize, opsonize, and eliminate bacterial invaders.