The classical pathway of activation of the complement system is a group of blood proteins that mediate the specific antibody response. It is triggered by antigen-bound antibody molecules. It is the binding of a specific part of the antibody molecule to the C1 component that initiates this pathway. The complement system is a complex biochemical cascade of the immune system, leading to cytolysis, chemotaxis, opsonization and inflammation, it can mark pathogens for phagocytosis. ... Schematic of antibody binding to an antigen An antibody is a protein used by the immune system to identify and neutralize foreign objects like bacteria and viruses. ... An antigen is a molecule that stimulates the production of antibodies. ...

This initial enzyme, C1, is a complex formed through a calcium-dependent association between two reversibly interacting subunits, C1q and C1r2s2. Approximately 70% of C1 is at all times present in this complex form. C1 occurs in serum as a proenzyme which tends to undergo autoactivation but which is strictly controlled by C1-inhibitor (C1-In or C1 esterase). Upon the binding of C1 to immune complexes by virtue of the affinity of C1q for immunoglobulins (specifically IgM and IgG), the controlling action of C1-In is overcome and C1q effects activation of C1r2s2. Neuraminidase ribbon diagram An enzyme (in Greek en = in and zyme = leaven) is a protein, or protein complex, that catalyzes a chemical reaction and also controls the 3D orientation of the catalyzed substrates. ... Calcium plays a vital role in the biochemistry of the cell, particularly in signal transduction pathways. ... C1-inhibitor (C1inh) is a serine protease inhibitor (serpin) protein, the main function of which is inhibition of the complement system. ... Schematic of antibody binding to an antigen An antibody is a protein complex used by the immune system to identify and neutralize foreign objects like bacteria and viruses. ...

C1q possesses no intrinsic catalytic activity, but when any of several activators bind to the C1q subcomponent of C1, the homologous C1r and C1s subcomponents are converted into catalytically active species, namely C1r* and C1s*, triggering the first step of the classical pathway of complement activation.

Thus, on binding to immune complexes through C1q, the subunits of C1 become firmly associated and autoactivation commences even in the presence of the Cl-In. Initially, a conformational change in C1r occurs, followed by proteolytic activation which results in the cleavage of all four polypeptide chains of C1r2s2. The two activated C1s subunits are then able to catalyse the assembly of the C3-convertase, C4b2a, which has been formed from C2 and C4. C3-convertase is the name of the enzyme composed of the C4b-C2b complex, which forms during the Classical pathyway of the complement system. ...

See Also

• Alternate complement pathway