Atopic eczema, Langerhans cells and allergy

While the pathomechanisms of respiratory atopy are rather well established, the role of IgE-mediated hypersensitivity in the elicitation and maintenance of eczematous skin lesions in atopic eczema is still controversial. Few diseases are characterized by an equally elevated production of IgE antibodies as atopic eczema. Many authors, however, regard this only as epiphenomenon. On the other hand, there is clearcut clinical evidence for exogenous elicitation of atopic eczema by contact with aero or food allergens. A variety of hypotheses may help to explain the participation of IgE antibodies in the induction of eczema: vasoactive mediators secreted by skin mast cells or basophils after allergen contact may produce itch, contact urticaria or a 'late-phase-reaction' with consequent eczematous skin changes further maintained by scratch responses. Recent investigations stress a possible role of Langerhans cells in the epidermis with a low affinity receptor for IgE with possible function for antigen presentation, mediator release or regulatory interactions. Certain cytokines such as interleukin-4 or gamma-interferon are able to enhance the expression of the IgE-receptor on the surface of Langerhans cells. IL-4 and gamma-interferon act synergistically in this respect on Langerhans cells, contrary to B cells. Furthermore lymphocytes may act directly via certain cytokines (e.g. histamine releasing factor, chemotactic factors etc.) on mast cells or eosinophil granulocytes in a proinflammatory sense. Eosinophils seem also to be involved in the inflammatory response in atopic eczema by releasing products such as major basic protein (MBP) or eosinophil cationic protein (ECP) which has been found to be elevated in severe atopic eczema.(ABSTRACT TRUNCATED AT 250 WORDS)