The thymus is the human immune organ where T cells mature.
Located behind the sternum in the upper chest, the thymus provides an environment in which immature lymphocytes develop into functional T lymphocytes. During this process, cells are tested for their ability to recognize foreign antigens while avoiding strong attacks against the body’s own tissues. This selection is central to immune tolerance.
T cells originate from precursor cells made in bone marrow, but they migrate to the thymus for the distinctive stages of their maturation. The organ is relatively large and active during childhood. After puberty, much of its functional tissue is gradually replaced by fat, a process called thymic involution, although thymic activity does not simply vanish at puberty.
The thymus should not be confused with the spleen. The spleen filters blood and helps coordinate immune responses to blood-borne material, whereas the thymus specializes in T-cell development. The thymus also produces signaling molecules that support immune-cell maturation.