Common signs of a mild allergic reaction include itching, nasal congestion, watery eyes, hives and rashes, according to MedlinePlus. The symptoms of a more severe allergic reaction may include abdominal pain, chest tightness, wheezing, dizziness or unconsciousness.
An allergic reaction to an insect bite often includes redness, pain and local swelling, explains WebMD. An allergic reaction to food may cause vomiting, stomach cramps and diarrhea. The most severe allergic reaction, anaphylaxis, can cause shortness of breath or wheezing, itching and hives all over the body, throat tightness or hoarseness, and tingling in the extremities, scalp or lips. Anaphylaxis can be deadly, so an individual with symptoms should have immediate medical care.
Allergic reactions generally occur soon after an individual is exposed to the allergen, explains MedlinePlus. For food allergies, the allergic reaction may occur several hours after eating. Often, the more an individual is exposed to the allergen, the more severe the allergic reactions become.
An allergic reaction occurs when an individual is exposed to an allergen and the body begins producing antibodies to bind the allergen, according to WebMD. The antibodies attach to certain blood cells, and the allergens bind to attachments of those blood cells. The blood cells react by releasing various chemicals, including histamine, into the bloodstream. Most allergic reaction symptoms are caused by histamine.