The role of antigen presenting cells at distinct anatomic sites: they accelerate and they slow down allergies.

It has been repeatedly demonstrated that allergic reactions are driven by the continuous flow of antigen uptake and presentation processes, which are perpetuated mainly by dendritic cells (DC). The ability of allergens to cause allergic inflammation is contingent upon the presence of an immunological milieu and microenvironment that either privileges Th2 responses or prohibits these reactions by the induction of contraregulatory anti-inflammatory activities of the immune system. In the light of recent developments it appears that DC have to manage two opposing tasks: on the one hand they can favor pro-inflammatory reactions and actively induce a T-cell response, yet on the other hand they serve an important function as 'silencers' in the immune system by sending out anti-inflammatory, tolerance inducing signals. This unique capacity of DC has opened several exciting possibilities for a role of DC in both - accelerating and slowing down allergic reactions. It is therefore a challenge to understand in which way DC subtypes located at distinct anatomic sites with frequent allergen exposure, such as the skin, the nasal mucosa, the respiratory tree or the mucosa of the intestinal tract can have an impact on mechanisms involved in tolerance induction or effective immunity.