thymus
Meaning of thymus: An organ that does most of its work before adulthood and then fades, having already trained the immune cells a person keeps for life.
Definition of thymus
The thymus is a soft organ in the upper chest, sitting behind the breastbone and in front of the heart, where T lymphocytes complete their development. Immature cells arrive from the bone marrow, and inside the thymus they are tested and selected so that the ones released into the blood recognize foreign material without attacking the body's own tissue.
What happens to a T cell inside the thymus?
Two rounds of selection, and most cells fail them. Each developing cell first assembles a receptor at random, which gives it a unique ability to recognize one particular shape. It is then shown fragments of the body's own proteins. A cell whose receptor cannot bind them at all is useless and is allowed to die, since a receptor that engages nothing will never recognize an infected cell either. A cell whose receptor binds them too strongly is dangerous and is destroyed or diverted into a regulating role. Only the small minority in between is released. This is where the distinction between self and foreign is largely built, and failures of the process contribute to autoimmune disease.
How does it differ from a lymph node?
| Feature | Thymus and bone marrow | Lymph nodes and spleen |
|---|---|---|
| Role | Where immune cells are made and matured | Where mature cells meet foreign material |
| Timing | Work is concentrated early in life | Active throughout life |
| What arrives | Immature precursor cells | Material drained from tissue, and circulating cells |
Because it produces and educates cells rather than responding to infection, the thymus is called a primary lymphoid organ, alongside the bone marrow. Nodes and spleen are secondary, and they are where a response by the immune system is actually mounted.
Why does it shrink with age?
The thymus is at its largest relative to the body during infancy and childhood, filling much of the space behind the breastbone, and it begins to shrink from around puberty. Working tissue is steadily replaced by fat, a normal change called involution. Output of new T cells falls sharply as this happens, but the pool of mature T cells built up earlier persists and renews itself within the tissues, so immunity does not disappear along with the organ. The change explains why a prominent thymus on a child's chest imaging is expected while a mass in the same position in an adult is not. The organ is also the site of thymoma, a tumor associated with the muscle weakness of myasthenia gravis, and it is absent or tiny in some inherited conditions that leave a child with almost no T cells.
Used in a sentence
Imaging of the chest incidentally revealed a normal appearing thymus in the anterior mediastinum.
Good to know
The thymus is most active earlier in life, so its gradual replacement by fatty tissue with age is an expected change rather than a sign of disease.