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Immune complexes and erythrocyte CR1 (complement receptor type 1): effect of CR1 numbers on binding and release reactions.

We performed experiments to investigate whether immune complexes opsonized with C3b and iC3b transferred from CR1 on one erythrocyte to CR1 on others, and studied the effect of variation in erythrocyte CR1 number on the transfer reaction. We used populations of cells of different blood groups to study this phenomenon which were separated by differential agglutination with monoclonal anti-group antibodies. The rate of transfer of immune complexes between erythrocytes was related to CR1 concentration of both donor and recipient cells; fastest transfer occurred from donor cells of low CR1 numbers to recipient cells of high CR1. These results were not explained by a difference in the binding constant of immune complexes to erythrocytes bearing different numbers of CR1. In the absence of factor I, complexes partitioned between erythrocytes according to their relative concentrations of CR1 with no release of complexes into solution. In serum, the proportion of complexes bound to donor and recipient erythrocytes was similarly related to their respective CR1 numbers with progressive release of complexes into solution. Erythrocyte CR1 may act as a dynamic buffering system which prevents immune complexes that have bound complement from fixing to vascular endothelium.