Deficiencies of the Complement System - Primary Immunodeficiencies

DEFICIENCIES OF THE COMPLEMENT SYSTEM

The complement system is an integral part of the immune system, and alterations in normal components of complement can result in increased susceptibility to infectious diseases and to immune-mediated disorders (Porth, 2002). Improved techniques to iden-tify the individual components of the complement system have led to a steady increase in the number of deficiencies identified. Dis-orders of the complement system can be primary or secondary.

C2 and C3 component deficiencies result in diminished resis-tance to bacterial infections. Angioneurotic edema is caused by an inherited deficiency of the inhibitor of C1 esterase, which op-poses the release of inflammatory mediators. A deficiency of this inhibitor results in frequent episodes of urticaria and edema in various parts of the body.

Patients with paroxysmal nocturnal hemoglobinuria (PNH) lack decay-accelerating factor (DAF), which is found on erythro-cytes (red blood cells). DAF normally protects the erythrocytes from lysis (disintegration). In PNH, the complement component C3b accumulates on the CR1 molecule on the erythrocyte, acts as a binding site for the late-acting component, and allows lysis to occur.