The thymus is a hugely important immune organ. In newborn babies and young children, the thymus is very large, however, as a person ages their thymus shrinks to almost non-existence by their late-teens. The epithelium of the thymus is called the thymic stroma - the cells bear surface markers that create a unique environment for young thymocytes that initiate their development of T cell-specific surface markers.
The thymus is the site of T cell maturation and development with thymocytes migrating to the thymus from the bone marrow soon after their production. While huge numbers of precursor T cells are generated in the thymus through proliferation, the vast majority will also die there due to tolerance testing failure.
The thymus is home to a large colony of resident specialized dendritic cells that solely serve to present antigens to developing T cells can assist in their maturation process. Presentation of antigen by dendritic cells here differentiates T cells from double-negative to double-positive thymocytes, preparing them of their first round of central tolerance self-recognition testing.