RBC adhesion is dependent on immobilized ligand density. Immobilized ligand density was varied to measure the effect on RBC adhesion in centrifugation. (A) Human RBCs adhere to human SIRPα1ex-coated wells with half-max adhesion at C₅₀ = 1670 sites/μm². Inset shows log scale with the average physiologic range of SIRPα1ex on various phagocytes, per Table 3. (B) Pig RBCs adhere to human SIRPα1ex-coated wells more tightly than human RBCs. (C) Human RBCs adhere more strongly to B6H12 antibody-coated wells than to human SIRPα1ex. Averages (± SEM) of % area covered by RBCs after centrifugation from multiple experiments are shown and the ligand density at half-maximal adhesion (C₅₀) was determined by data fit. Note that SIRPα1 immobilized here prevents clustering and limits avidity effects, consistent with the low Hill coefficients. Error bars represent plus or minus 1 SEM from multiple experiments.