Allergy is characterized by an overreaction of the human immune system to a foreign protein substance "allergen" that is eaten, breathed into the lungs, injected or touched. This immune overreaction can results in symptoms such as coughing, sneezing, itchy eyes, runny nose and scratchy throat. In severe cases it can also result in rashes, hives, lower blood pressure, difficulty breathing, asthma attacks, and even death.
The immune system normally protects the body against harmful substances, such as bacteria and viruses. It also reacts to foreign substances called allergens. These are usually harmless and in most people do not cause a problem. In a person with allergies, the immune response is oversensitive. When it recognizes an allergen, the immune system launches a response. Chemicals such as histamines are released. Allergens that you breathe in often cause a stuffy nose, itchy nose and throat, mucus, cough, and wheezing.
Eating something are allergic to can cause nausea, vomiting, abdominal pain, cramping, diarrhea, or a severe, life-threatening reaction. Allergens that touch the skin can cause a skin rash, hives, itching, blisters, or skin peeling. Drug allergies usually involve the whole body and can lead to a variety of symptoms. The best way to reduce symptoms is to avoid what causes allergies. This is especially important for food and drug allergies.