It is a fatty bilobate gland at the root of the neck and in the front part of the chest . It is known to be large in infancy and then to diminish steadily until a remnant remains in adults. It has now been demonstrated that the thymus is necessary in the late stages of embryonic development and in early infancy to develop the immunity mechanisms. The thymus is of utmost importance in the body's defenses against infection , for in the first few weeks of life the lymphocytes produced in it migrate into the bloodstream and colonize lymph nodes all over the body.
Lymphocytes manufacture antibodies and are vital for immunity .
If the thymus of newborn animals is damaged, there is a marked stunting of growth , development is retarded and there is a great susceptibility to infections - due to the reduction of the number of lymphocytes and impairment of antibody formation. The reduced resistance to infection which is brought about by corticosteroid treatment is paralleled by a shrinkage of the thymus and lymphoid tissue generally. This will lead ultimately to a lowered capacity for antibody formation. The thymus usually consists of two lobes, but they may unite to form a single lobe or may have an intermediate lobe between them. It is situated in the upper chest cavity along the trachea , overlapping the great blood vessels as they leave the heart.