Basic components of the immune system part 69

Classical pathway

As its name implies, this is the usual pathway whereby antigen-antibody complexes in the presence of complement destroy the invading organism. The antibody (either IgM or IgG) causes a conformational change in the Fc portion of the antibody to reveal a binding site for the first complement C1q. This component consists of six subunits and reacts with the Fc   via its globular heads. The activation of this component requires the binding of two globular heads for activation. Thus, one molecule of IgM with its pentameric conformation can easily activate C1q, while the ability of IgG, which has only two sites to activate C1q, is low. IgA, IgD and IgE do not activate the classical pathway.  C1q in turn activates C4 and C2, generating the complex C4b2b, which is the C3 "convertase" of the classical pathway. After the splitting of C3 into C3a and C3b is achieved, C3a possesses anaphylatoid and chemotactic activity. However, more important is C3b, which forms the complex C3b4b2b, which is the C5 convertase and initiates the final "lytic" attack complex.