An allergy occurs when the immune system reacts to substances (allergens) that are generally harmless and in most people do not cause an immune response. Antibodies are formed by the immune system in response to the presence of an antigen such as pollen, mold, dust or dander.
Review Date 2/27/2018
Updated by: Stuart I. Henochowicz, MD, FACP, Associate Clinical Professor of Medicine, Division of Allergy, Immunology, and Rheumatology, Georgetown University Medical School, Washington, DC. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.