The Effect of C4 on C1 Binding and Activation

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Abstract
Cell-bound human C4 [complement component 4] enhances the uptake of C.hivin.1 [activated C1] by Ig-carrying cells. The effect is more pronounced with small amounts of IgG than with large amounts. The presence of C4 on the cell surface dramatically promotes the enzymatic effect of C1 on C2. The C1 binding is efficiently blocked, and the C1 activation is almost abolished by pretreating IgG-bearing cells with protein A from Staphylococcus aureus. Protein A has no effect on cells carrying C4 in addition to IgG. Protein A added in small amounts to IgG-coated sheep erythrocytes inhibits the hemolysis by subsequently added human serum, whereas even larger amounts of protein A has no effect when added to cells carrying C4. Cells with and without C4 take up protein A equally well.