Contact dermatitis
This is not an IgE-mediated allergy but a cell mediated allergic reaction. Contact allergy develops in some people after frequent or regular contact of skin or mucosa with specific substances that are generally harmless in most people. The contact dermatitis takes about 24 to 72 hours to develop and is associated with itch. The cells involved in the reaction are predominantly mononuclear cells specifically cytotoxic T-lymphocytes and macrophages. The Epicutaneous Patch test is the only test available for detection of the trigger factors in contact allergy. The test is carried out directly on the skin. In the patch testing various standard dilutions of the allergens are placed in contact with the skin for a period of 48 to 72 hours. In an individual with contact allergy an itching eczematous eruption (vesicles, erythema, papules) appear localized to the spot where the allergen comes in contact with the skin. The substance causing the reaction is identified and avoided. The area of severe dermatitis corresponds to the site of most intense contact with the allergen. Some areas include the hands (hairdressers, florists, food handlers, costume jewellery handlers or wearers and leather glove wearers), face (fragrances, preservatives, hair colours, dyes and permanent waves), eyelids (nail polish, eye cosmetics, and their applicators, eyelash curlers, perfume, formaldehyde and plants), body trunk (zippers and buttons, rubber in elastic undergarments, fragrances in soaps used, skin care products, detergents, formaldehyde, textile resins, and dyes), and legs (nylon stocking dye, rubber in support hose, shoe constituents and medicaments).

Infants can develop food allergy through contact with the food. The IgE test may be negative but patch test based on foods will reveal a positive reaction in such cases. This test is called Atopic Patch Test and the immune reaction is caused by cytotoxic T-cells. About 25% of children suffer from contact allergy to foods.