Although the human IgG heavy‐chain constant domains have greater than 90% identity, key amino acid differences confer each subclass with unique structural and functional properties. IgG1 and IgG3 are “universal” ligands, that is, they bind to all FcγRs. Formal measurement of the weak, micromolar KD interactions of the low‐affinity receptors with monomeric IgG1 also revealed differing affinities between the low‐affinity FcγRs, with inhibitory FcγRIIb generally having the lowest affinity and FcγRIII the higher, sometimes referred to as a “moderate” affinity receptor.7, 61