Someone said the thymus is like T cell school, and the little T lymphocytes are sent there where they begin in the outer cortex, and as they go through school over the years, they move on down to the deeper cortex and eventually graduate into the medulla and leave via efferent venules and lymphatic vessels. The purpose of T school is to train T cells in different vocations, such as helpers, killers, and antigen recognizing ones. Here's the the brutal part, if you don't pass the test (and you can't recognize your body's antigens), you die (95%). However, once you leave, you are skilled at whatever you have been programmed to do.