Repeated allergen challenge as a new research model for studying allergic reactions.

Repeated allergen challenge might be a more relevant model for studying symptomatic disease, because it gives more information on the patient's handling of chronic allergen exposure. Thus, this experimental model has more resemblance to the situation of natural allergen exposure, and the allergen load can be standardized. However, the response to repeated allergen challenge in individual patients can show a large variation, that is from a strongly enhanced response to complete diminution of the response. Successful allergen immunotherapy can change the response pattern of repeated allergen challenge in the skin into down-regulation of the late reaction. Chronic or repeated allergen exposure may result in an enhanced allergen-specific allergic response, involving allergen-specific T-cell activation. Different subsets of T cells can exert either activating or suppressive effects on inflammatory cells involved in subsequent allergic reactions. CD8+ T cells might exert suppressive effects, because they seem to be associated with a subsequent down-regulation of the late skin reaction after repeated allergen challenge (Fig. 4). Further studies are needed to compare the responses to repeated allergen challenge with the response to natural seasonal allergen exposure in the same patients and to explore possible underlying mechanisms using, for example, nasal biopsies.