What Is a Delayed Allergic Reaction?

Delayed allergy; delayed hypersensitivity; delayed type hypersensitivity, also known as "delayed hypersensitivity", belongs to the type IV allergy in the four-type classification proposed by Gell & Coombs, belonging to the cell Immunity (because it is not mediated by antibodies, but by cells) is a hypersensitivity response mediated by T lymphocytes. It occurs without the participation of antibodies or complements, and forms sensitized lymphocytes under the action of allergens. When it is contacted again with the same allergen, it can show a kind of slowness (the reaction only occurs after at least about 12 hours, 24-72 Peak hours), local allergic inflammation characterized by mononuclear cell infiltration and cell degeneration and necrosis. Such as contact dermatitis, transplant rejection, tissue damage caused by Mycobacterium tuberculosis, BCG vaccination and so on.