FcγRIII forms are two highly related gene products, FcγRIIIa and FcγRIIIb. The FcγRIIIa is expressed on NK cells and professional phagocytes, particularly macrophages. It is only recently apparent that FcγRIIIa is expressed on neutrophils, albeit at low levels, but plays a role in their function.23 FcγRIIIb is unique to humans and unlike other FcγRs it is attached to cell membrane via a glycophosphatidyl anchor. It is expressed, predominantly and abundantly, on human neutrophils.7 Its effector function depends in part on its coexpression with FcγRIIa. The lack of FcγRIIIb on macaque neutrophils appears to be compensated for by an increase in FcγRIIa expression.15