CD4 and infection: Seminal studies on experimental infections of Leishmania major in C57BL/6 and BALB/c inbred mouse strains correlated parasite control with the elaboration of interferon gamma (IFN-γ) by CD4+ T cells and uncontrolled infections with the absence of IFN-γ in the C57BL/6 and BALB/c mice, respectively (Scott et al., 1989; Reiner and Locksley, 1995).