hypersensitivity state mediated by T lymphocytes. When the antigen is introduced locally, eg in the skin, a gradual local accumulation of T-cells and monocytes results. The visible reaction is reddening and local swelling, increasing for 24-48 hr and then subsiding , sometimes leaving a small scar due to necrosis of blood vessels. Tuberculin testing is a good example. Frequently this and other types of hypersensitivity co-exist, and reactions are not clear cut.