Complement Deficiencies: Loss of Opsonization, Chemotaxis, and Lytic Functions

Core Idea

Early complement deficiencies (C1, C2, C4) impair immune complex clearance and are associated with SLE. Terminal component deficiencies (C5-C9) increase susceptibility to Neisseria infections. Factor H or Factor I deficiency causes atypical HUS through uncontrolled C3 activation.

Explainer

The complement system you studied is a cascade of proteins that amplifies the immune response through three key functions: opsonization (coating pathogens for phagocytosis via C3b deposition), chemotaxis (recruiting neutrophils and macrophages via C3a and C5a anaphylatoxins), and direct lysis (forming the membrane attack complex, or MAC, from C5b-9). When any component of this cascade is genetically absent or dysfunctional, the downstream effects follow directly from which function is lost — and the clinical phenotype maps predictably onto the cascade architecture.

Early pathway deficiencies (C1q, C1r, C1s, C4, or C2) abolish classical pathway activation. The classical pathway is normally triggered by antibody-antigen complexes, and its early components handle the critical task of clearing immune complexes from the circulation. When C1 or C4 is absent, immune complexes accumulate — and the body's mechanism for tolerating self-antigens is disrupted. This is why C2 deficiency (the most common complement deficiency in Western populations) and C1/C4 deficiencies are strongly associated with a lupus-like syndrome: uncleared immune complexes deposit in glomeruli, joints, and skin, mimicking systemic lupus erythematosus (SLE). The lesson is that complement isn't just offensive (killing pathogens) — it performs housekeeping that prevents autoimmunity.

Terminal component deficiencies (C5 through C9) specifically impair MAC formation while leaving opsonization and chemotaxis intact. The clinical consequence is narrow but memorable: patients cannot lyse gram-negative diplococci efficiently. Neisseria species — both *N. meningitidis* (meningococcus) and *N. gonorrhoeae* — are unusually resistant to phagocytic killing and depend on MAC-mediated lysis for clearance. Terminal complement-deficient patients therefore suffer recurrent, often severe Neisseria infections, sometimes with unusual serotypes. This specificity underscores a general principle: different pathogens exploit different vulnerabilities in the immune defense architecture.

The regulatory complement proteins tell a third story. Factor H and Factor I are brakes on the alternative pathway — without them, C3 is constitutively activated and rapidly depleted. Uncontrolled C3 consumption leaves insufficient complement for normal immune function, but the more immediate danger is that activated complement turns destructive. Atypical hemolytic uremic syndrome (aHUS) results when unregulated complement activation attacks endothelial cells in small vessels, particularly the renal microvasculature. Thrombotic microangiopathy develops: microthrombi form in glomerular capillaries, red blood cells are sheared as they pass through (microangiopathic hemolytic anemia), and renal failure ensues. This is mechanistically distinct from typical HUS caused by Shiga toxin; the trigger here is dysregulated complement, not exogenous bacterial toxin. Recognizing this distinction matters clinically because atypical HUS can be treated with eculizumab, a monoclonal antibody blocking C5 — a targeted therapy that would be irrelevant in toxin-mediated HUS.