The public sometimes incorrectly uses the word allergy to refer to a disease or almost any unfavorable or adverse reaction. We frequently hear someone say, “I have allergies,” or “She’s allergic to everything”. In reality, allergies are reactions that are usually caused by an overactive immune system. These reactions can occur in a variety of organs in the body, resulting in diseases such as asthma, allergic rhinitis and eczema.
Our immune system is made up of a number of different cells that come from organs throughout the body; principally from bone marrow, the thymus gland, and a network of lymph nodes and tissues scattered throughout the body, including the spleen, gastrointestinal tract, tonsils and the adenoid (an olive-shaped structure that is located at the top of the throat behind the nose).
The immune system protects the body against disease by singling out and destroying foreign invaders, such as viruses and bacteria. In an allergic reaction, the immune system overreacts by producing antibodies called Immunoglobulin E (IgE). These antibodies travel to cells that release chemicals, causing an allergic reaction against a normally harmless substance, such as pollen or animal dander (material shed from and animal’s body). These allergy-provoking substances are called allergens. This reaction usually causes symptoms in the nose, lungs, throat, sinuses, ears, lining of the stomach or on the skin.
Each type of IgE has specific ‘radars’ for each type of allergen. That's why some people are only allergic to house dust mites, for example (they would only have the IgE antibodies specific to house dust mites); while others experience allergic reactions to multiple allergens because they have many more types of IgE antibodies.
It is not yet fully understood why some substances trigger allergies and others do not, nor why some people have allergic reactions while others don’t. A family history of allergies is the single most important factor that puts you at risk of developing allergic disease.