The immune system is of crucial importance in defense against infection. It has to cope with a large number of different pathogens that relentlessly develop new ways to avoid recognition or elimination. Yet most infections are cleared. Immune-system genes must evolve to keep pace with increasingly sophisticated evasion by pathogens. In this article we examine features of human defense genes that reflect the demands imposed by such intense selection. Key examples are MHC and KIR genes, where features such as polygeny and polymorphism facilitate the comprehensive logistics needed to counteract infection.