The immune system recognizes and responds to almost any foreign molecule; it cannot discern between molecules that are characteristic of potentially infective agents and those that are not. In other words, an immune response can be induced by materials that have nothing to do with infection. The mechanisms brought into play, though beneficial for eliminating microbes, are not necessarily beneficial when otherwise innocuous substances are targeted. Furthermore, even initially protective mechanisms can cause secondary disorders when they operate on too great a scale or for a longer period than necessary, thereby damaging tissues remote from the infection. The terms allergy and hypersensitivity are commonly used to describe inappropriate immune responses that occur when an individual becomes sensitized to harmless substances.

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