This happens during organ rejection, and in rheumatoid arthritis and some forms of diabetes. The job of the immune system is to distinguish what belongs in the body from what doesn't. The bulk of this work is carried out by white blood cells known as lymphocytes. B cells develop in bone marrow and produce antibodies. Each B cell can recognize only a single antigen.
T cells mature in the thymus gland, which lies on the heart.
For a long time, researchers thought the immune system couldn't distinguish between normal and cancerous cells. Today, they know that cancer cells produce abnormal proteins that can be recognized as antigens. Cancer vaccines are not used to prevent disease, as are vaccines for infectious diseases. Rather, they are given to help stop a cancer from recurring. Cancer vaccines usually consist of killed cells or parts of cells from the patient's tumor.