An antibody is a protein produced by the body's immune system when it detects harmful substances, called antigens. Examples of antigens include microorganisms (such as bacteria, fungi, parasites, and viruses) and chemicals.
Antibodies are also produced when the immune system mistakenly considers healthy tissue a harmful substance. See: Autoimmune disorders
Each type of antibody is unique and defends the body against one specific type of antigen.
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., Inc.