IFNG and infection: When vaccinated mice are secondly challenged with wild-type parasites, the immune system rapidly recognizes and produces high levels of IFN-γ and other pro-inflammatory cytokines to activate cellular immune responses, and simultaneously, the high level of T. gondii-specific IgG antibodies hinders the parasite attachment and invasion to host cells and activates the classical complement pathway to clear secondary infections efficiently in cooperation (Sayles et al., 2000; Spellberg and Edwards, 2001; Pifer and Yarovinsky, 2011; Hunter and Sibley, 2012).