Once created in response to an antigen, an antibody continues to resist that foreign molecule. This allows our bodies to fight off diseases that previously afflicted us, without getting sick again. For example, most people will only contract chickenpox once in their lifetime, no matter how often they are exposed to the disease. This phenomenon is also what makes vaccinations possible. A vaccine is an injection of killed or weakened virus that stimulates the production of antibodies when introduced into the body.
Antibodies have many useful purposes, in addition to protection against disease.
They can be used to diagnose a wide variety of illnesses, as well as to detect the presence of abnormal substances in the blood, such as drugs, viruses, and bacterial products. In order to be used for diagnostic and detection purposes, antibodies that bind specifically to the virus, bacteria or drug of interest are necessary. When an antigen is introduced into the body, however, a variety of different antibodies are synthesized. The antigen binds to receptors on the surface of B cells, stimulating those cells to produce antibodies specific to the antigen. When seeking out its target, the antibody binds to a site on the antigen known as the epitope.