The complement system is a cascade of serum proteins that amplify inflammation, tag pathogens for destruction (opsonization), and directly lyse cells via the membrane attack complex. Three activation pathways (classical, alternative, lectin) converge at C3 activation. Complement bridges innate and adaptive immunity by amplifying responses to antibodies and pathogen surfaces.
From your study of innate immunity, you know that the body has rapid, non-specific defenses against pathogens. The complement system is one of the most powerful of these defenses — a set of over 30 soluble proteins circulating in the blood, mostly produced by the liver, that form an enzymatic cascade capable of destroying pathogens, recruiting immune cells, and amplifying the overall immune response. Think of it as a molecular alarm and weapon system that is always loaded and ready to fire, requiring only the right trigger to activate.
The system operates through three activation pathways that differ in how they are triggered but converge on the same central event. The classical pathway is initiated when the C1 complex (C1q, C1r, C1s) binds to antibodies (IgG or IgM) that are already attached to a pathogen surface — this is the direct link to adaptive immunity. The lectin pathway is triggered when mannose-binding lectin (MBL) recognizes mannose-rich carbohydrate patterns on microbial surfaces — these sugar patterns are common on bacteria and fungi but rare on human cells. The alternative pathway is constitutively active at a low level through spontaneous hydrolysis of C3 (called "tickover") and amplifies on any surface that lacks the regulatory proteins found on host cells. All three pathways converge at the cleavage of C3 into C3a and C3b — the central amplification step of the entire cascade.
Once C3b is generated, three major effector functions follow. First, opsonization: C3b deposits covalently on the pathogen surface, coating it with molecular "eat me" signals that phagocytes (neutrophils and macrophages) recognize through complement receptors, dramatically enhancing phagocytosis. Second, inflammation: the small fragments released during cleavage — C3a, C4a, and especially C5a — act as anaphylatoxins, potent inflammatory mediators that recruit neutrophils, increase vascular permeability, and stimulate mast cell degranulation. C5a is one of the most powerful chemoattractants known. Third, direct lysis: downstream components C5b through C9 assemble on the pathogen membrane to form the membrane attack complex (MAC), a ring-shaped pore that punctures the lipid bilayer, disrupting osmotic balance and killing the cell.
The complement system is extraordinarily powerful, which is why it is tightly regulated by host proteins. Factor H and Factor I inactivate C3b on host cell surfaces, CD59 (protectin) prevents MAC assembly on self-cells, and C1 inhibitor controls the classical and lectin pathways. Deficiencies in these regulators cause serious diseases: paroxysmal nocturnal hemoglobinuria (loss of CD59) leads to complement-mediated destruction of the patient's own red blood cells. Understanding complement is essential for immunology because it sits at the intersection of innate and adaptive immunity — it can be triggered independently of antibodies (alternative and lectin pathways) or recruited by antibodies (classical pathway), making it a versatile effector system that you will encounter repeatedly in topics from antibody function to transplant rejection.