Allergy <!-- google_ad_client = "ca-pub-2327677141881672"; /* Short title Links */ google_ad_slot = "3055805829"; google_ad_width = 468; google_ad_height = 15; //-->

An unusual sensitivity to a substance normally harmless to others. The host mounts an immunologic response against an allergen and reacts whenever exposed to it. Usually caused by a foreign antigen in food, pollen or chemicals.