IFNG and infectious disease: Interferon-gamma (IFN-gamma) produced by antigen-specific T cells plays a key role in activation of anti-viral and anti-tumor immune responses, and it has been used as a possible surrogate marker of protection against infectious diseases and tumor development (16, 17).The administration of chicken IFN-gamma with MDV vaccine increases vaccine efficacy, suggesting that IFN-gamma may play an important role in providing protection against MD (18, 19).