Thymus

The thymus is a primary lymphoid organ located in the upper anterior mediastinum, behind the sternum. It is most active during childhood and adolescence, where it is the site of T lymphocyte (T cell) maturation and selection. Thymic epithelial cells present self-antigens to developing T cells, eliminating self-reactive ones (negative selection) and promoting the survival of useful ones (positive selection). It undergoes involution with age.