Allergic reactions are caused by harmless allergens, which are recognized by the specific immune system. Allergen-specific T cells are assumed to play a key role in the sensitization phase and in immunological memory. Current immunological concepts suggest that asymptomatic T-cell memory cells also exist, tagging the allergen as harmless and preventing an inappropriate response and thus allergic symptoms. Proinflammatory T cells mediate allergic inflammation by exceeding the induction of IgE and competing with other T-cell subsets. Therefore, molecular mechanisms leading to pro- or anti-inflammatory T-cell memory cells appear as the key mechanism in allergy.