Allergy Causes Make an Appointment Refer a Patient Ask a Question Reviewed by Carah B. Santos, MD (November 01, 2016) Allergies and allergic reactions occur when the immune system misidentifies harmless foreign substances, called allergens, and reacts to them as if they were harmful. Specifically, allergy symptoms result when the immune system makes IgE antibody that binds to a specific allergen in an effort to protect us from otherwise harmless allergens. Allergens can include things like pollen, animal dander, latex and foods. How People Develop Allergies To become allergic, a person must have a genetic predisposition for allergy, meaning having a personal or family history of allergies, and must be exposed to a sensitizing allergen. Most allergies begin in early childhood, and early environmental factors play a major role in the development of allergies. Primary Exposure - Sensitization An allergen, such as mold spores or pollen, enters the body through the mouth or nose or lands on the skin. In a person with allergies, white blood cells (T cells) recognize the allergen as foreign and release chemicals in response. These chemicals travel through the blood and instruct another kind of white blood cell (B cells) to produce IgE antibodies. Some of these IgE antibodies attach to the outside of another special type of white blood cell called a mast cell. Mast cells are scattered throughout the skin and respiratory tract. Their purpose is to help mediate the inflammatory response of the immune system. IgE antibodies can remain attached to mast cells for many years. Secondary Exposure - The Cause of Allergy Symptoms When the same allergen is encountered again, it binds directly to the IgE antibody attached to the outside of mast cells. This causes the mast cells to become activated and release chemicals, such as histamine. Histamine is an example of a chemical mediator — a special "messenger" chemical that immune cells use to talk to each other. Histamine is one of the best-known mediators — it is responsible for causing many of the symptoms associated with allergies. Histamine opens small blood vessels, causing them to leak fluid. This results in inflammation: warming and swelling of the skin, itching and watery eyes. Histamine causes sneezing and the increased mucus production in the nasal cavity and airways that leads to runny nose, postnasal drip and cough. Why Allergy Is Selective Some people are allergic to lots of things. Some people are allergic to only one or to very few things. Learn more about why allergy is selective. View Allergy Home Allergy Symptoms Clinical Trials For more than 100 years, National Jewish Health has been committed to finding new treatments and cures for diseases. Search our clinical trials.