Hence, half of the millions of lymphocytes produced every minute in the bone marrow go to the thymus gland​—a small gland located behind the breastbone—​for their training as T cells. Concerning this, the book The Body Victorious says: “The lymphocytes which attend the technical college of the thymus are the helper, suppressor, and killer cells called T-lymphocytes (or T-cells). They are among the most indispensable armed forces of the immune system.”

To help us absorb the magnitude of what the immune system is accomplishing, an article in the National Geographic, June 1986, details the problem confronting the thymus gland: “Somehow, as the T cells mature in the thymus, one learns to recognize the antigens of, say, the hepatitis virus, another to identify a strain of flu antigens, a third to detect rhinovirus 14 [a cold virus], and so on.” After commenting on the “staggering task the thymus confronts,” the article says that in nature there are “antigens in hundreds of millions of different shapes. The thymus must turn out a group of T cells that recognizes each one. . . . The thymus pumps out T cells by the tens of millions. Even though only a few of them may recognize any one antigen, the collective scouting force is vast enough to identify the almost infinite variety of antigens nature produces.”