Allergic reactions result when a person's immune system reacts to normally harmless substances in the environment. Substances that cause allergic reactions are known as allergens.
Allergic reactions are common. Many allergic reactions are mild, while others can be severe and even life-threatening. Common symptoms of mild allergic reactions include itching, hives, nasal congestion, and red, watery eyes. Symptoms of moderate to severe allergic reactions include abdominal pain, abnormal breath sounds, anxiety, chest discomfort or tightness, cough, diarrhea, nausea or vomiting, swelling of the face, eyes or tongue, and flushing or redness of the face. Anaphylaxis is a sudden and severe allergic reaction that occurs within minutes of exposure to the allergen. Immediate medical attention is needed for this condition.