An antigen is a substance (such as part of a virus or bacteria)
that triggers the immune system to produce cells (antibodies) that attack and
try to destroy the antigen. The body usually produces one specific type of
antibody for each antigen.
Sometimes the body mistakenly recognizes its own tissue as an
antigen or foreign substance and produces antibodies against its own tissue.
This is called an autoimmune response or autoimmune
E. Gregory Thompson, MD - Internal Medicine & W. David Colby IV, MSc, MD, FRCPC - Infectious Disease