The thymus is a useless vestigial organ, especially after childhood.

The thymus was a difficult organ to understand. It is largest and most active early in life, then shrinks substantially with age. Removing it from adult animals often appeared to have little immediate effect.

That made it easy to dismiss the organ as vestigial or even as a place where lymphocytes simply went to die.

Jacques Miller changed the interpretation in 1961 by removing the thymus from newborn mice. The animals developed severe defects in immune function and became unusually susceptible to infection.

The timing explained the earlier confusion. The thymus is especially important while the immune system is being built. Developing lymphocytes enter the organ, where T cells—immune cells central to targeted adaptive responses—undergo a training and selection process. Cells capable of useful responses are preserved, while many that react strongly against the body’s own tissues are removed or restrained.

Adults retain long-lived T cells generated earlier, so removing an adult thymus cannot reproduce the consequences of losing it during development.

The thymus does involute with age, but reduced size does not mean absence of function. Its developmental role is central to adaptive immunity and immune tolerance.