Note especially the Thymus:
"The thymus is an important lymphatic organ. Found in front of your trachea (windpipe), its main role is to teach white blood cells to recognise our own cells. In order for the immune system to function properly, white blood cells must be able to discriminate between invading pathogens and the body's own cells. After T cells are produced in the bone marrow they migrate to the thymus. Here they are educated by the thymus to stop them from attacking our own cells. It is thought that some forms of autoimmune disease (where the body attacks itself) may be due to problems with this process. The thymus is at its largest during puberty, and gets smaller as you age. "