A glandular body of obscure function (one of the so-called ductless glands) situated near the base of the neck in vertebrate animals; in people it usually disappears after the period of childhood.

This organ is important in the development of the immune response in the newborn. Its removal during early chilhood has been associated with an increased susceptibility to acute infectious diseases at a later time.

The thymus is essential to the maturation of the thymic lymphoid cells, called T cells. When the T cells enter the circulation, they are the small-sized and medium-sized lymphocytes which may survive for up to five years. These cells are important in the body's cellular immune response.