The vertebrate body is an ideal breeding ground for viruses and provides the conditions that promote their growth, survival, and transmission. The immune system evolved and deals with this challenge. Mutually assured destruction is not a viable evolutionary strategy; thus, the study of host-virus interactions provides not only a glimpse of life at immunity＇s edge, but it has also illuminated essential functions of the immune system, in particular, the area of major histocompatibility complex-restricted antigen presentation.