To eradicate Brucella infection at the late stage of infection, adaptive immunity triggers three major responses including activation of brucellacidal activities in macrophages by IFN-γ stimulation, killing of Brucella-infected immune cells by cytotoxic T lymphocytes – a main source of IFN-γ secretion, and opsonization of the pathogen by Th1- type antibody isotypes IgG2a and IgG3 [34, 35]. This evidence concerns the gene IGHG3 and infection.