CD4⁺ helper T cells mediate resistance to tuberculosis, presumably by enhancing the antimicrobial activity of macrophages within which the Mycobacterium tuberculosis organism grows. A first step in resistance should be the presentation of mycobacterial antigens by macrophages to CD4⁺ T cells. However, when the antigenic stimulus is limited to organisms growing in human monocytes, the organisms become sequestered from immune CD4⁺ T cells. This block in presentation is selective for growing mycobacteria and not for other stimuli. Sequestration would allow replicating organisms to persist in infected individuals and may contribute to virulence.