This pathway is initiated by reaction with antigen-antibody complexes. The final lytic form of activated complement is the result of a series of enzymatic cleavages and recombination of cleavage products. It contains complement components C1, C4, and C2.
C1 esterase inhibitor inactivated the protease activity of C1 and is deficient in hereditary angioedema. Angioedema is acute edema of cutaneous or mucosal structures, most commonly involving the lips and eyelids. Laryngeal edema can occur and be life-threatening. Serum C4 is low, and other complement components, such as C3, are consumed.
C2 deficiency is the most common complement deficiency and is associated with severe pyogenic respiratory infections and systemic lupus erythematosus (SLE).