A 32 year old man presents with a 3-month history of weight loss, night sweats, a productive cough with blood-tinged sputum, anorexia, general malaise, and a low grade fever. A PPD skin test shows > 10 mm of induration. If the area of induration were biopsied, which of the following type of reactive cells would be found?

The CD4+ population of T lymphocytes, specifically TH1 cells, are responsible for the delayed hypersensitivity reaction seen with a skin test in a previously sensitized patient. The clinical pattern in the test question is classic for reactivation or adult-type tuberculosis. B lymphocytes are involved in humoral immune reactions. Antibody production is not a feature of tuberculin hypersensitivity. Eosinophils are impoant in type I hypersensitivity reactions and in immune mediated responses to parasitic infections (ADCC). They are not associated with tuberculin hypersensitivity. Mast cells are tissue cells which are involved in type I hypersensitivity reactions. They have surface receptors for the Fc fragment of the IgE molecule.