The immune system must recognize millions of potential antigens. There are fewer than 30,000 genes in the human body, so it is impossible to have one gene for every antigen. Instead, the DNA in millions of white blood cells in the bone marrow is shuffled to create cells with unique receptors, each of which can bind to a different antigen. Some receptors bind to tissues in the human body itself, so to prevent the body from attacking itself, those self-reactive white blood cells are destroyed during further development in the thymus, in which iodine is necessary for its development and activity.