Immunization with irradiated sporozoites can protect both experimental animals and humans against subsequent infection with live parasites (reviewed in 5,8,9,10) and this protective effect, at least in part, is accounted for by the activity of antigen-specific CD8+ T-cells [11,12,13,14,15,16,17,18], which prevent the development of parasites in the liver of the infected host. Here, CD8A is linked to infection.