Some allergens trigger an allergic reaction on first contact, but some are slow acting. Allergy symptoms may only begin to appear after several days of smelling, eating, or coming into contact with an allergen. In other cases, a person may have been in daily contact with a substance (such as tobacco) for a long time without any problems, and then suddenly develop an allergy to it without apparent cause.
有些過敏原一接觸就激發反應，有些過敏原卻有延遲性，可能要聞過、吃過或觸摸過好幾天後，才慢慢出現過敏現象；也有些以往天天接觸都相安無事的物質（如香煙），卻不明原因地忽然誘發了過敏......。