Viral diseases pose major threats to humans and other animals, including the billions of chickens that are an important food source as well as a public health concern due to zoonotic pathogens. Unlike humans and other typical mammals, the major histocompatibility complex (MHC) of chickens can confer decisive resistance or susceptibility to many viruses. Examples are Marek's disease virus (MDV) and Infectious bursal disease virus (IBDV). We used a new in vitro infection system and immunopeptidomics to identify peptides presented to T lymphocytes via classical MHC class II molecules.