Although the role of CD8 + T cells during primary immune responses is controversial, these cells appear to play a prominent role in protecting mice from a secondary challenge (Muller et al., 1993 . Indeed, antigen-specific CD8 + T cells were expanding up to 50-fold in the spleen and lymph nodes of reinfected BALB/c mice . This expansion correlated with a substantial production of IFN-γ, which is thought to be essential for controlling Leishmania infections. These observations have major implications for vaccine design.