The measurement of avian cellular immunity is critical to understanding the role and regulation of avian lymphocytes following avian influenza (AI) virus infection. Although the ability to measure avian T cell responses has steadily increased over the last few years, few studies have examined the role of cell-mediated immunity in avian species against the AI virus. Because of the structural and functional differences between mammalian and avian immune systems—including MHC architecture, different modes of somatic recombination for antibody production, and the absence of lymph nodes in birds—the extent to which birds and mammals regulate similar immune responses against the AI virus is currently under investigation. The increasing availability of monoclonal antibodies recognizing avian T cell-associated antigens as well as a number of inbred lines of chickens with genetically defined MHC haplotypes make this an important field of research for the future.