An antibody is a type of protein. The body's immune system produces antibodies when it detects harmful substances, called antigens. Examples of antigens include microorganisms (such as as bacteria, fungi, parasites, and viruses) and chemicals.
Antibodies are also be 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.