A delayed-type hypersensitivity reaction is a specific type of allergic response that distinguishes itself from most allergies by not manifesting immediately upon exposure to an allergen. Unlike immediate allergic reactions, delayed-type hypersensitivity reactions occur in individuals who have become sensitized to a typically harmless substance, known as an allergen.
Delayed allergies, also known as type IV hypersensitivity reactions, can occur as a result of various infections caused by bacteria, viruses, fungi, or protozoa. They may also arise following vaccination with a live virus vaccine. Examples of delayed allergic reactions include contact dermatitis, characterized by skin inflammation upon contact with certain substances, as well as the immune response triggered by tuberculosis infection.