A. gp120-coated beads were incubated with b12 variants triple, double, standard and lala (Fc binding knockout) at varying concentrations (100, 20, 5 ng/ml), and the phagocytic score was determined. B. Receptor blocking experiments in which either FcγR2 or FcγR3 was blocked by preincubation with an anti- FcγR antibody. Results are presented as the ratio of the phagocytic score for the variant:double mutant at each given condition.