When irritants are applied to the scalp they are expected to induce an allergic reaction. This allergic reaction draws the T lymphocytes away from the follicle in order to deal with the irritant. The hair follicle then has a chance to regrow new hair. It is theorized that this how irritants work.

Study involving irritants.

STUDY: Allergic and irritant contact dermatitis compared in the treatment of alopecia totalis and universalis. A comparison of the value of topical diphencyprone and tretinoin gel.