Another characteristic of the allergic contact dermatitis rash is that it usually appears after a day or two after exposure to the allergen, unlike irritant contact dermatitis that appears immediately after the contact with the trigger. Other symptoms may include itching, skin redness or inflammation, localized swelling and the area may become more tender or warmer. If left untreated, the skin may darken and become leathery and cracked. Pain can also be present. Dermatitis can occur anywhere on the skin, but is most common on the hands (22% of people), scattered across the body (18%), or on the face (17%).The rash and other symptoms typically occur 24 to 48 hours after the exposure; in some cases, the rash may persist for weeks. Once an individual has developed a skin reaction to a certain substance it is most likely that they will have it for the rest of their life, and the symptoms will reappear when in contact with the allergen. Cause
Common allergens implicated include the following: 

Bacitracin – topical antibiotic found by itself, or as Polysporin or Triple Antibiotic
Balsam of Peru (Myroxylon pereirae) – used in food and drink for flavoring, in perfumes and toiletries for fragrance, and in medicine and pharmaceutical items for healing properties; derived from tree resin. It may also be a component of artificial vanilla and/or cinnamon flavorings. Chromium – used in the tanning of leather. Also a component of uncured cement/mortar, facial cosmetics and some bar soaps.
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 sensitisation by ACD-associated allergens (1986).