A pair of naturally occurring antibodies may dampen complement-dependent phagocytosis of red cells with a positive antiglobulin test in healthy blood donors

Background and Objective It is known that red blood cells (RBC) from healthy blood donors with a positive direct antiglobulin test (DAT) for IgG continue to circulate despite carrying elevated numbers of IgG molecules. To unravel the properties of these RBC‐bound IgG, we studied them not only on whole RBC populations, but also on density‐fractionated RBCs. Materials and Methods The properties of acid‐eluted RBC‐bound IgG and plasma IgG were studied by ELISA for binding to RBC proteins and opsonins, and by blotting. In vitro phagocytosis was studied on density‐separated RBCs. Results IgG‐DAT‐positive blood donors carried most IgG molecules on dense RBCs and had more RBCs of high density than DAT‐negative controls. Their densest RBCs were older than the oldest RBCs of DAT‐negative controls, based on the band 4·1a/b ratio. In vitro phagocytosis of senescent RBCs from IgG‐DAT‐positive donors was 1·5 to 2 fold higher than that of senescent control cells, but the same or less in the presence of physiological IgG concentrations, implying that RBC‐bound IgGs impaired complement‐dependent uptake. The IgG molecules on these DAT‐positive RBCs comprised anti‐band 3 naturally occurring antibodies (NAbs) and were two‐ to fivefold enriched in anti‐C3 and framework‐specific anti‐idiotypic NAbs as compared to controls. Correspondingly, anti‐C3 and framework‐specific anti‐idiotypic NAbs were proportionally elevated in the plasma of two‐thirds of DAT+ donors. Conclusions Extra‐binding of anti‐C3 together with anti‐idiotypic NAbs to senescent RBC‐associated C3 fragments may suppress complement‐dependent RBC phagocytosis and may prolong the in vivo life span of RBCs.