(self-tanners, shampoo, nail polish, sunscreen) and in industrial products (polishes, paints and waxes). Sap from certain species of mangrove and agave Soluble salts of platinum – see platinosis Thiomersal – mercury compound used in local antiseptics and in vaccines Topical anesthetics – such as pramoxine or diphenhydramine, after prolonged use Topical steroid – see steroid allergy Urushiol – oily coating from plants of Toxicodendron genus – poison ivy, poison oak, and poison sumac. Also found in mango plants, mango skin, cashews, and smoke from burning urushiol-containing plants, which can cause skin as well as severe lung irritation. Mechanism ACD arises as a result of two essential stages: an induction phase, which primes and sensitizes the immune system for an allergic response, and an elicitation phase, in which this response is triggered. As it involves a cell-mediated allergic response, ACD is termed a Type IV delayed hypersensitivity reaction, making it an exception in the usage of the designation "allergic," which otherwise usually refers to type I hypersensitivity reactions. Contact allergens are essentially soluble haptens (low in molecular weight) and, as such, have the physico-chemical properties that allow them to cross the stratum corneum of the skin. They can only cause their response as part of a complete antigen, involving their association with epidermal proteins forming hapten-protein conjugates. This, in turn, requires them to be protein-reactive. The conjugate formed is then recognized as a foreign body by the Langerhans cells (LCs) (and in some cases other Dendritic cells (DCs)), which then internalize the protein; transport it via the lymphatic system to the regional lymph nodes; and present the antigen to T-lymphocytes. This process is controlled by cytokines and chemokines – with tumor necrosis factor alpha (TNF-α) and certain members of the interleukin family (1, 13 and 18) – and their action serves either to promote or to inhibit the mobilization and migration of these LCs. As the LCs are transported to the lymph nodes, they become differentiated and transform into DCs, which are immunostimulatory in nature. Once within the lymph glands, the differentiated DCs present the allergenic epitope associated with the allergen to T lymphocytes. These T cells then divide and differentiate, clonally multiplying so that if the allergen is experienced again by the individual, these T cells will respond more quickly and more aggressively. White et al. have suggested that there appears to be a threshold to the mechanisms of allergic