Which statement is true about Type IV hypersensitivity?

Which statement is true about Type IV hypersensitivity?

• A. It is an immediate reaction occurring within minutes
• B. It is a delayed-type hypersensitivity that can cause edema and induration at the exposure site
• C. It involves immune complex deposition in vessels
• D. It is mediated by IgE antibodies

Answer: (B)

Type IV hypersensitivity is a delayed-type reaction driven by T lymphocytes rather than antibodies. After exposure and sensitization, T cells release cytokines that recruit and activate macrophages at the exposure site, producing inflammation that peaks around 24 to 72 hours later. The result is edema and induration from local cellular inflammation, which is why this statement is true. This contrasts with immediate, minutes-range reactions that involve IgE-mediated mast cell degranulation (Type I), and with immune complex–mediated damage in vessels (Type III). It also highlights that IgE antibodies are not the mediators in Type IV. Examples include contact dermatitis from poison ivy and tuberculin skin testing, where the reaction develops after a delay due to T cell–driven inflammation.

Type IV hypersensitivity is a delayed-type reaction driven by T lymphocytes rather than antibodies. After exposure and sensitization, T cells release cytokines that recruit and activate macrophages at the exposure site, producing inflammation that peaks around 24 to 72 hours later. The result is edema and induration from local cellular inflammation, which is why this statement is true. This contrasts with immediate, minutes-range reactions that involve IgE-mediated mast cell degranulation (Type I), and with immune complex–mediated damage in vessels (Type III). It also highlights that IgE antibodies are not the mediators in Type IV. Examples include contact dermatitis from poison ivy and tuberculin skin testing, where the reaction develops after a delay due to T cell–driven inflammation.