Complement Immunology Review 385
Definition
Functions
Classical pathway*
Alternative pathway*
Lectin pathway*
Present in highest
concentration in
plasma
Key component of
all pathways
Ions required
Group of >30 proteins involved in phagocytosis & clearance of foreign antigens. Most are inactive enzyme
precursors that are converted to active enzymes in precise order (cascade).
Inflammation, opsonization, chemotaxis, cell lysis.
Triggered by ag/ab rxn. IgM is most efficient activator. Single molecule attached to 2 adjacent ags can
initiate cascade. IgG1, 2, & 3 can activate complement but at least 2 molecules required.
Recognition unit: C1 (first to bind).
Activation unit: C4, C2, C3.
Membrane attack complex: C5, C6, C7, C8, C9 (cell lysis).
Ab independent. Activated by bacteria, fungi, viruses, tumor cells, some parasites.
Ab independent. Initiated by mannose-binding lectin (MBL). Nonspecific recognition of sugars on microor-
ganisms. Important defense mechanism in infancy.
C3
C3
Ca2+, Mg2+
continued...