The tuberculin test:

Sensitization during prior exposure to Mycobacterium tuberculosis results in production of memory Th1 cells to Mycobacterial proteins. When purified tuberculin is injected into the skin, memory Th1 cells secrete cytokines to attract macrophages and granulocytes and cause induration and erythema (firm red swelling). Delayed hypersensitivity reactions to tuberculin begin at 5 to 6 h and reach peak at 48 to 72 hours. Initial phase involving uptake, processing, and presentation by Langerhans-type dendritic cell in skin to T cells locally and in paracoical zones of nearby lymph nodes. T lymphocyte secretes cytokines, especially IFNgamma; macrophages are recruited and activated resulting in induration and erythema. The infiltrate of the tuberculin reaction is composed of monocytes (80-90%), CD4 T cells, CD8 T cells, and Langerhan cells. The lesion usually resolves within 5-7 days but if there is persistence of antigen, it may develop into a granulomatouS reaction. Ref: Tuberculin test: Microbiology notes of Sridhar Rao P.N.