What type of hypersensitivity reaction is shown by a positive tuberculin test?

• A. Type I
• B. Type III
• C. Type IV
• D. Type II

Answer: (C)

The tuberculin skin test demonstrates a delayed-type hypersensitivity, a cell-mediated immune response. In people who have been exposed to TB, memory T cells recognize the purified protein derivative and release cytokines that recruit and activate macrophages at the injection site. This leads to local inflammation and induration that becomes evident after about 48 to 72 hours. Since this reaction is driven by T cells rather than antibodies, it fits Type IV hypersensitivity. By contrast, Type I is immediate and antibody-mediated (IgE), Type II involves antibodies targeting cells, and Type III involves immune complex deposition, none of which describe the tuberculin reaction.