As already noted, more recent studies employing Leishmania major parasites 22 and mycobacteria 23,24 show that infection with low numbers can generate potent and exclusive Th1 responses, without the production of detectable antibody, whereas infection with higher, but still relatively low, numbers generates predominant Th1 responses, with the generation of a few Th2, IL-4-producing CD4 T cells, and predominant production of IgG2a antibodies 22–24. This evidence concerns the gene CD4 and infection.