What is allergy?
Allergy is an immunological hypersensitivity mediated by immunoglobulin E antibody (IgE). It is not a disease itself, but a mechanism leading to diseases such as rhinoconjunctivitis, urticaria, asthma and anaphylaxis. A normally harmless substance - like pollen, food or cat saliva - will cause the immune system to defend the body against it. In an allergic reaction the mast cells release a chemical called histamine, which is the primary cause for the allergic symptoms.
Allergies can be seen in many organs, but most commonly they affect the skin and mucous membranes, as these are the barriers between the body and the outside environment. Pollen allergy causes itching in the eyes and a runny nose. Contact allergies can induce a rash. Food allergies cause itching in the mouth as well as abdominal pain and vomiting. The most severe allergic reaction is anaphylaxis. It can rapidly lead to a life-threatening condition where blood pressure drops and breathing may be obstructed because of throat swelling.
Allergy often starts at an early age. In most cases it persists through the life, but the symptoms may decrease, and some people outgrow their allergy entirely.
Sometimes other reactions are incorrectly referred to as allergy. For example, irritating or toxic substances can cause symptoms in the skin or abdomen that resemble an allergic reaction. Occasionally, sensitivity to certain foods, such as lactose intolerance, is also being called allergy. However, only the immune-mediated hypersensitivity is true allergy.
Allergy starts with a sensitization phase that doesn't yet cause allergic symptoms, but wires the immune system to recognize the allergen. The actual allergic reaction is launched upon the next encounter of the allergen and every time after that.