The thymus is part of the lymphoid system and is therefore an organ of the immune system located at the base of the neck under the breastbone. The thymus consists of two large lobes that atrophy during adolescence. The lymphoid tissue gradually gives way to fatty tissue.

The thymus is a primary lymphoid organ (together with the bone marrow), and is therefore involved in the differentiation and selection of T cells and therefore plays a role in immunity. The thymus plays a very important role in the development of the immune system in children by ensuring the maturation of T lymphocytes, also called thymocytes. This process is called thymic development. The maturation of prothymocytes into T lymphocytes takes place during the migration of cells from the cortex to the medulla and under the influence of two hormones synthesised by the thymus, thymopoietin and thymulin. The lymphocytes then acquire TCR receptors and CD4 and CD8 coreceptors, which are involved in antigen recognition.