Control Mechanisms In Thealternate Pathway

Similar proteins control the classical and alternate pathways. Factor H competes with factor B for binding to C3b and accelerates the decay of the alternate C3 convertase C3Bbb (Fearon et al., 1975). In addition, factor H cleaves and inactivates C4b when cofactors such as CR1 and MCP are present (Fig. 9).

Binding of the cofactors is dependent on the concentration of sialic acid residues on the cell surface. High concentrations of sialic acid on the cell surface increase the binding ofFactor H and other cofactors. Low concentrations of sialic acid favor the binding ofFactor B and potentiation of the alternate complement activation pathway. Other protein accelerating decay proteins (e.g. Factor H) inhibitthe binding of C3B to Bb (Muller-Eberhard, 1971, 1975). FactorHbinds

Beta 1H

Figure 9. Control proteins in alter- —— native pathway. _

strongly to highly sulfated glycoconjugates such as dextran sulfate, heparin, chondroitin sulfate A, and carrageenan. Binding depends on the number, orientation, and polymeric arrangement of sulfate groups (Men and Pangburn, 1994).

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