An antigen is a substance that can provoke an immune response. Typically antigens are substances not usually found in the body.
Update Date 8/30/2012
Updated by: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.