Immune responses that give rise to the transition from acute, severe disease to inapparent infection are only partially defined for T. equi. The development of specific immunoglobulin isotypes (IgG 1, 4, 7) correlates with control of parasitemia during the acute phase of infection, and additional isotypes, IgG3 and IgG5 (IgG(T)), appear after the resolution of parasitemia [10,11]. This evidence concerns the gene IGHG3 and parasitic infectious disease.