The allergy is an immune system disease. This one reacts in a disproportionate way to some usually harmless substances like dust, mites, pollen, etc. The allergic person has an high concentration of dedicated antibodies IgE in the blood. When the allergic person comes into contact with the guilty substances, some symptoms turn immediately up, like skin reaction or swelling. In some cases severe symptoms, like anaphylactic shock, can turn up. If you suspect to suffer of an allergy, making a dedicated test is high recommendable.