In support of the hypothesis that genetic effects on both CD8+ (class I) and CD4+ (class II) cells modify the predisposition to infections, it should be noted that some infectious diseases, such as HIV, HBV, HCV, and leprosy, show associations to more than one of the classic MHC classes and, in some cases, the associations differ between populations (Table 2). This evidence concerns the gene CD4 and infection.