Thymus, Where Art Thou?

The thymus, located between your lungs and behind your sternum, is where T-lymphocytes develop. Though these white blood cells begin from stem cells in bone marrow, they further specialize into T-lymphocytes here. The "T" actually stands for "thymus" to reflect this origin. The thymus is an interesting organ: Although it's active in young people and teenagers, it shrinks and becomes much less active in adults.