The role of complement in immune complex-mediated pathological disorders

It is shown that activation peptides of complement such as C3b or C4b play an important role in the processing and removal of immune complexes. Solubilization of immune complexes occurs as the result of the accumulation of complement peptides in the lattice, and they interfere by sterically inhibiting the binding of antigen to antibody. Furthermore, phagocytic cells possess receptors for Fc and C3, and the ingestion of antibody sensitized particles depends on the interaction between these receptors and the particle-associated ligands C3b and IgG. These receptors have synergistic roles in phagocytosis: C3 receptors promote adherence of sensitized particles to phagocytes, and Fc receptors promote ingestion of the particles. The important role of complement in the clearance of immune complexes is further demonstrated by observations that a defect in clearance occurs in diseases such as malaria in which an increased consumption of complement leads to hypocomplementemia, or in genetic deficiencies in certain components of complement such as C1q, C1r, C2, C3, C4, or C1̄-INA. The significance of complement in immune complex diseases is discussed, and some of the methods used in the detection of immune complexes are described.