What occurs when there is a second exposure to an allergen?

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Upon a second exposure in someone who is sensitized, the immune system responds with a rapid, IgE-mediated reaction. IgE antibodies are bound to mast cells and basophils from the first exposure. When the allergen reappears, it cross-links these bound IgE molecules, triggering degranulation and the release of histamine and other inflammatory mediators. This causes quick, widespread inflammation—vasodilation, increased vascular permeability, swelling, and smooth muscle contraction—which is why the reaction can be described as massive. Tolerance isn’t typical in an allergic individual’s response to a known allergen, delayed-type hypersensitivity involves a different mechanism (T-cell–mediated and slower, like contact dermatitis), and a reduced inflammation scenario would contradict the amplified response expected after sensitization.