The failure of MVA85A against TB despite its induction of CD4+ Th1 immunity, the observation that BCG-specific CD4+ and CD8+ T-cell responses did not correlate with protection against TB disease in one study (95) together with the limited results achieved by current leprosy vaccines, clearly underline the need for a better understanding of the host mechanisms that are responsible for protection against both TB and leprosy. Here, CD4 is linked to tuberculosis.