Role of C4-binding protein in the defective lysis of sensitized sheep erythrocytes by mouse complement.

Passage of mouse EDTA-plasma over an anti-C4-binding protein (anti-C4-bp) affinity column reduced the C4-bp level by an average of 34% and increased the classical C pathway activity as measured with sensitized normal and desialylated sheep erythrocytes, but not versus sensitized rabbit erythrocytes. This suggests that C-regulation by C4-bp is independent of membrane sialic acid and provides evidence for the presence of a hitherto unknown preferential site for mouse C4-bp on sheep erythrocytes. The absence of both this site and sialic acid on their membranes makes sensitized rabbit erythrocytes useful target cells for assay of mouse classical C pathway activity.