IMMUNOLOGY

Detailed explanation-1: -Complement is a system of plasma proteins that can be activated directly by pathogens or indirectly by pathogen-bound antibody, leading to a cascade of reactions that occurs on the surface of pathogens and generates active components with various effector functions.

Detailed explanation-2: -It is well known that both IgG and IgM can activate complement via the classical pathway by binding of C1q to the Fc regions of these immunoglobulins. Recent advances have shown that also IgA is capable of activating the complement system.

Detailed explanation-3: -Historically, complement’s only established role in adaptive immunity was its function in humoral antibody response, in which complement opsonization reduces the B cell activation threshold in response to antigens, subsequently leading to increased antibody production.