Which receptor on phagocytes has the highest affinity for binding IgG?

The receptor on phagocytes that has the highest affinity for binding IgG is FcR1 (also known as CD64). This receptor is a high-affinity receptor for the Fc portion of IgG antibodies, meaning that it binds IgG more tightly compared to other Fc receptors. This strong interaction is crucial for mediating the immune response, as it enhances the phagocytic activity of the cells. When phagocytes express FcR1, they can efficiently ingest pathogens coated with IgG, promoting opsonization and leading to the clearance of these pathogens from the body. This attribute makes FcR1 vital in mounting an effective immune response, particularly in dealing with infections.

In contrast, the other Fc receptors mentioned have lower affinities for IgG. FcR2a and FcR2b (CD32a and CD32b, respectively) are lower-affinity receptors and play different roles in modulating immune responses. FcR3 (CD16) also binds IgG but with lesser affinity than FcR1. Each of these receptors contributes to the immune system in specific ways, but FcR1 stands out due to its high affinity for binding IgG antibodies.