Receptor activities for cell-bound degradation products of the third complement component, C3b and C3d, were demonstrated on cells from nine cultured human lymphoid lines and spleen lymphocytes from eight strains of mice. These receptor activities were found with rosette formation and immune adherence assays by utilizing sensitized sheep erythrocytes coated with C3 derived from human, mouse, guinea pig, and rabbit serum. Some cultured human lymphoid cells displayed receptor activities for both C3b and C3d, others showed only the one for C3d, and two cell lines reacted negatively. All murine cells examined showed receptor activities for murine C3d and to a lesser extent for human C3d together with receptor activity for guinea pig C3b; however, human C3b-carrying cells could not be attached to murine lymphocytes. Sensitized sheep erythrocytes coated with C1-C5 of rabbit serum deficient in C6 were found to react much like erythrocytes coated with human C3b and thus provided a useful reagent for the detection of C3b receptor activities on human lymphoid cells. There was no correlation between the expression of C3b and C3d receptors and the H-2 or HL-A phenotype or the antigenic determinants of β2-microglobulin which suggests that these complement receptors have unique chemical structures representing additional and useful cell surface markers.
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