Allergic contact dermatitis is a classical type IV delayed hypersensitivity immune response. This cell-mediated response is also known as hapten-type delayed hypersensitivity. Allergic contact dermatitis may be viewed as hyperreactivity of the skin immune system. In the present view of allergic contact dermatitis, individuals are born in a state of tolerance to environmental haptenic allergens. During life, sensitization to any hapten(s) may occur. Subsequent elicitation of a sensitized individual then leads to dermatitis, often accompanied by severe pruritus. Human epidermal Langerhans cells play a central role during the sensitization stage. These antigen presenting dendritic cells, loaded with environmental haptens, continuously leave the epidermis through the lymph vessels and, upon arrival in the paracortical T-cell areas of the skin draining lymph nodes, they differentiate into interdigitating cells. There is now in-vitro evidence for such a maturation of human Langerhans cells into interdigitating cells. Any given individual may be sensitized to any particular hapten by this route. Allergic contact dermatitis is probably a skin-specific disease because of the capacity of Langerhans/interdigitating cells to induce relatively naive T-cells to become memory T-cells. Factors determining the ultimate outcome in this continuous hapten presenting process, i.e. whether or not the original state of tolerance will persist, are still enigmatic. During elicitation, when the allergenic hapten is applied epicutaneously to a sensitized individual, a focal accumulation of immune response associated cells producing a wide variety of cytokines and inflammatory mediators ultimately results in the clinical condition of allergic contact dermatitis. Langerhans cells do not seem to play a major role during this stage.(ABSTRACT TRUNCATED AT 250 WORDS)