+An **allergy** is an overreaction by the [Immune System](/wiki/immune_system) to a normally harmless substance, known as an [Antigen](/wiki/antigen). This mistaken threat triggers a defense response, leading to a range of symptoms.
+## See also
+- [Anaphylaxis](/wiki/anaphylaxis)
+- [Asthma](/wiki/asthma)
+- [Histamine](/wiki/histamine)