CD8A and immunodeficiency disease: This concept of “cancer immune surveillance” was first recognized in cases of immunodeficiency in which immunocompromised patients or mice presented with higher risks of developing tumors.31,32 Subsequent studies showed that tumor antigen-specific CD8+ T cells infiltrate tumor sites where they selectively kill cancer cells.3–5 Activation of CD8+ T cells requires two signals from antigen-presenting cells: the tumor-specific antigen and co-stimulatory molecules.