CD4 and tuberculosis: In a study of 30 patients with TB at different stages of treatment and 31 control healthy individuals with no signs or symptoms of TB but various degrees of exposure (household contacts, health care workers in a lung hospital, medical students, and unexposed healthy blood donors), it was shown that the percentage of PDD-specific CD4+ T-cells lacking the surface receptor CD27 (a state associated with advanced differentiation due to increasing TB exposure) is highly discriminating between active and latent TB infections [36].