Immunoferitin analysis of membrane antigen density: A. young and old human blood cells B. developing erythroid cells and extruded erythroid nuclei

Two immunoferritin techniques, (a) ferritin‐conjugated antibodies and (b) hybrid antibodies, were used to determine the surface antigen density on separated human young and old erythrocytes and on differentiating rabbit erythroid cells. The alterations of surface antigenicity caused by treatment of erythrocytes with neuraminidase were similarly studied. Old untreated cells have about a 25 % higher labeling density as compared with the young ones. The same age groups when treated with neuraminidase show a 45 % increase in labeling density as compared with untreated cells, but without significant differences between young and old cells. Counts performed on cell membranes labeled with ferritin‐conjugated antibodies or with hybrid antibodies revealed very similar results with both techniques. A progressive increase in labeling density, reaching a maximum at the orthochromatic erythroblast, follows the erythroid cells' division. A slight decrease in antigen density is observed on the reticulocyte. Some cross‐reaction with leukoid precursors is also observed. A possible correlation between the reduction of negative surface charge and the increase in membrane antigen density is pointed out. Expelled erythroid nuclei, surrounded by a narrow rim of cytoplasm and membrane, are more heavily labeled than other erythroid cells of the bone marrow. The increase in antigen density on the membrane surrounding the nucleus is already apparent when the nucleus is partially expelled. The alterations in the surface antigen density on old or neuraminidase‐treated erythrocytes and on expelled erythroid nuclei are attributed to the appearance of antigens previously masked by the neuraminic acid. The possibility that the increased membrane antigen density on the surface of old erythrocytes and expelled erythroid nuclei, as well as the reduction in negative surface charge, reflects factors determining the recognition of deteriorated membranes by macrophages and leading to their sequestration is discussed.