Review Question - QID 100467

This patient has symptoms and clinical findings consistent with tuberculosis infection of the lungs. Mycobacterium tuberculosis can initiate a TH1 response, leading to granuloma formation.

Tuberculosis, which most often affects the lungs and leads to respiratory impairment, is caused by the acid-fast bacterium, M. tuberculosis. The bacterium is particularly virulent due to its ability to prevent phagosome-lysosome fusion after engulfment by macrophages. Since a phagolysosome is not formed, the bacterium is able to escape degradation. However, when the adaptive immune system is activated, TH1 cells secrete IFN-gamma, which overcomes this bacterial defense mechanism. IFN-gamma also causes macrophages to differentiate into epithelioid cells, which may then combine to create large, multi-nucleated, giant cells (Langhans cells). This eventually leads to the formation of the granuloma that is characteristic of a tuberculosis infection.

Hauck et al. discuss the identification and management of patients with latent M. tuberculosis infection. They note that screening should be performed on high-risk populations, including "immigrants; residents and employees of congregate living facilities; and persons infected with human immunodeficiency virus." The most effective screening method remains the tuberculin skin test, although several newer tests are currently in development.

Cain et al. perform an epidemiological analysis to determine the percentage of patients who are diagnosed with M. tuberculosis in the United States that are foreign-born. The study included nearly 47,000 cases of M. tuberculosis and found that 57% of diagnoses were in patients who were born outside of the United States. They conclude that the yield of finding and treating latent TB may be increased in patients from Southeast Asia and Subsaharan Africa.

Figure A depicts an acid fast stain showing M. tuberculosis in pink. Figure B shows an radiograph of a patient with a right apical lung granuloma due to M. tuberculosis infection. Illustration A depicts the transitions between infection, latent TB, and active TB. Illustration B depicts the process of infected dendritic cells presenting antigens to Th1 cells in the lymph nodes, leading to Th1 cell entry into the lungs and activation of macrophages.

Incorrect Answers:
Answers 1-3, 5: These cells do not mediate granuloma formation in M. tuberculosis infection.