Non-clinical sciences/Immunology · Hypersensitivity

Type IV hypersensitivity

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What it means

Delayed, T-cell mediated (48–72 h) — sensitised CD4 T-cells release cytokines → macrophage activation; no antibody. Examples: tuberculin (Mantoux) reaction, contact dermatitis (nickel), granuloma formation (TB, sarcoid), chronic transplant rejection, T1DM β-cell destruction.

Worked example

A nickel watch-strap rash and the 72-h Mantoux induration are both delayed T-cell reactions.

In the exam

Induration appears at a tuberculin skin-test site only after 48–72 hours, mediated by sensitised T-cells, not antibody.

What settles it

Type IV is delayed (days) and cell-mediated; types I–III are antibody-mediated and faster.

Classically confused with

Type I hypersensitivity

Source: StatPearls