primary tuberculosis

In countries where bovine tuberculosis and infected milk have been eliminated, primary tuberculosis almost always begins in the lungs.

Typically, the inhaled bacilli implant in the distal airspaces of the lower part of the upper lobe or the upper part of the lower lobe, usually close to the pleura. As sensitization develops, a 1- to 1.5-cm area of gray-white inflammatory consolidation emerges, known as the Ghon focus. In most cases, the center of this focus undergoes caseous necrosis.

Tubercle bacilli, either free or within phagocytes, drain to the regional nodes, which also often caseate. This combination of parenchymal lung lesion and nodal involvement is referred to as the Ghon complex. During the first few weeks, there is also lymphatic and hematogenous dissemination to other parts of the body.

In approximately 95% of cases, development of cell-mediated immunity controls the infection. Hence, the Ghon complex undergoes progressive fibrosis, often followed by radiologically detectable calcification (Ranke complex), and despite seeding of other organs, no lesions develop.

Histologically, sites of active involvement are marked by a characteristic granulomatous inflammatory reaction that forms both caseating and noncaseating tubercles.

Individual tubercles are microscopic; it is only when multiple granulomas coalesce that they become macroscopically visible. The granulomas are usually enclosed within a fibroblastic rim punctuated by lymphocytes. Multinucleate giant cells are present in the granulomas.

Immunocompromised people do not form the characteristic granulomas