Decay Accelerating Factor and the Defect of Paroxysmal Nocturnal Hemoglobinuria

Upon activation of the complement cascade by a target of immune attack, C4b and C3b fragments associate covalently with molecules on the target's surface (reviewed in References 1 and 2). Once bound, the C4b and C3b polypeptides serve as anchors for the assembly of the C3 convertases, C4b2a and C3bBb. Amplification of C3 activation by these central enzymes of the cascade allows progression of the complement sequence and permits the formation of ligands which facilitate interaction of the target with host inflammatory cells (reviewed in Reference 3). The deposition of C4b and C3b on target surfaces is thus a key step in target processing and disposal.