Immune complex alterations occur on the human red blood cell membrane.

Experimental antigen-antibody complexes (Ag-Ab) were incubated at 37 degrees with human red blood cells (RBC) suspended in autologous normal serum and the reaction stopped after progressively increasing times. Bound antigen-antibody-complement complexes (Ag-Ab-C) were eluted from C3b receptors and the eluted Ag-Ab-C re-incubated with different blood cell types suspended in serum, or centrifuged (along with unbound Ag-Ab-C found in the serum) through 20-50% sucrose gradients. Ag-Ab-C recovered from C3b receptors shortly after initial binding to RBC bound efficiently to other RBC, polymorphonuclear and mononuclear cells, and sedimented rapidly. Ag-Ab-C simultaneously present in the serum sedimented with a similar velocity. Ag-Ab-C recovered at a subsequent time during RBC interaction bound less well to each blood cell type, and sedimented less rapidly. Decreased amounts of rapidly sedimenting Ag-Ab-C were present in the serum. Ag-Ab-C recovered from C3b receptors at a still later time in the course of RBC interaction bound poorly to each cell type, and sedimented slowly. Increased amounts of slowly sedimenting Ag-Ab-C were found in the serum. These findings indicate that alterations in properties of immune complexes can occur while they are associated with C3b receptors on RBC membrane in solid phase.