Disclosed are crystals, crystal structure Fc.gamma.RIIa protein, three dimensional coordinates of Fc.gamma.RIIa protein, and structures and models derived from the Fc.gamma.RIIa structure. Also disclosed are crystals of Fc.epsilon.RI protein and three dimensional coordinates of Fc.epsilon.RI protein monomers and dimers derived from the Fc.gamma.RIIa structure. Also disclosed are three dimensional coordinates of Fc.gamma.RIIIb proteins and models of Fc.gamma.RIIIb derived from the Fc.gamma.RIIa structure. The present invention also includes methods to produce such crystals, crystal structures and models. Uses of such crystals, crystal structures and models are also disclosed, including structure based drug design and therapeutic compositions.