Human red cells (E) were specifically and singly coated with either C4, C3b, or C3d by using human serum as the source of complement, and the amount of each component fixed was measured quantitatively by antiglobulin consumption tests. The immune adherence (IA) of E-C4, E-C3b and E-C3d to peripheral blood monocytes and polymorphonuclear leukocytes (PMN) was dependent upon the amount of the component on the red cell surface. The amount of adherence per molecule of component fixed was slightly greater with E-C4 than with E-C3b and much less than either with E-C3d. However, E-C3d adherence was more stable to washing or mechanical disruption than E-C3b adherence; E-C4 adherence was less stable than either. Unlike E-C3b when incubated with heated serum, resulting in E-C3d with continued IA activity, E-C4 incubated with serum completely lost IA activity.

51Cr-labeled E-C4, E-C3b and E-C3d were not lysed during incubation with monocytes even though they readily adhere to monocytes.

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