Hypersensitivity reaction to deodorant. View of a red rash (erythema) in the armpit (axilla) of a man caused by hypersensitivity to deodorant. A hypersensitivity reaction is the overreaction of the immune system to an antigen (protein recognis- ed as foreign). Hypersensitivity reactions only occur after previous exposure to the antigen, when the immune system has already been sensitised. Treatment involves taking corticosteroids (anti- inflammatory drugs) & painkillers; in addition, further exposure to the offending allergen should be avoided.