The pathogenesis of Tuberculosis is highly complex .

1) In healthy Individuals , after development of Primary complex , It is the cell mediated immunity which protects the individual from progression to TB disease . In a healthy individual , the tuberculin test suggests that the subject has TB infection & not TB disease . Only 5 - 10% progress from TB infection to TB disease .  The TB organisms are not totally destroyed , but only contained & can be reactivated any time .

2) In Immuno-competent Individuals , it is the delayed or cell mediated hypersensitivity , in which the macrophages , stimulated by the T-cells , which causes the tissue damage leading to bilateral upper lobe cavities & fibrotic lung disease . This can affect other organs such as lymph nodes , pleura , bones & joints , nervous system & abdomen . There is a switch from cell mediated immunity to delayed type or type IV hypersensitivity . Even in HIV patients , with CD 4 > 350 , this would be the presentation of TB disease & is called co-infection .

3) In Immuno-suppressed HIV patients , where the CD4 T cells are destroyed , The pathogenic TB disease , is bilateral lower lobe , non cavitating , interstitial lung disease , presenting as a miliary TB disease . The protection of cell mediated immunity is not available . In HIV individuals , 80% progress from TB infection to TB disease . Therefore , it is difficult to assume that there are no toxins or enzymes of the bacteria , which causes the disease . This is the presentation of TB disease in HIV patients with CD 4 < 200 , suggesting an opportunistic infection .

Both CD4 cells and TNF alpha are important in the pathogenesis of TB. In healthy individuals who are exposed to TB, a high concentration of TNF alpha is important in controlling the disease and producing granulomas. The organisms are not necessarily killed and can stay alive in a latent phase.

Once a patient becomes immunosuppressed with a CD4 count of