How is the classical complement pathway initiated?

The classical complement pathway begins when antibodies IgM or IgG bind to antigens, forming an immune complex. This binding causes a change in the Fc portion of the antibody, exposing the complement binding site. Subsequently, C1q attaches to the immune complex, initiating a series of activations involving C1r and C1s in a sequential manner. The process is initiated when C1q attaches to antibodies bound to antigens, activating C1r and C1s and inducing the cleavage of C4 and C2. Once activated, C1s proceeds to trigger the cleavage of subsequent components, C2 and C4, resulting in the formation of C4b2a, known as the classical pathway C3 convertase. This C4b2a complex plays a crucial role in the complement cascade. The subsequent phase involves the formation of C4b2a3b, known as the C5 convertase, which leads to the generation of C5b. C5b then initiates the assembly of the terminal lytic complement complex, also known as membrane attack complex (MAC).