IGHG3 and viral infectious disease: Equine IgG3/5 responses are Th2-associated (25, 26) and have been described in humoral responses to Tetanus toxoid (40), helminths (42), and allergic responses in Culicoides hypersensitivity (17), while equine IgG4/7 is Th1-associated (25) and is induced in immune responses to virus infection and challenge (26).