Cells important to the immune response that mature in the thymus are known as

Germ Cell Tumors

Any type of germ cell tumor can occur in the anterior mediastinum. The gross appearance is similar to that seen in tumors arising in the testes.

Lymphomas

Hodgkin disease and non-Hodgkin lymphomas can occur at this site. They usually present as lobulated fleshy masses.

History

Until 1960 the function of the thymus was obscure. In fact even in the 1950s it was not appreciated that the thymus is normally a very large organ in infancy. With the availability of chest X-rays, pediatricians were impressed with the size of the thymus and invoked the condition of ‘status thymolymphaticus’ – chronic strangulation by an enlarged thymus – to explain minor respiratory ailments. To relieve ‘pressure’ on the trachea, the thymus was exposed to irradiation! Despite such bizarre reasoning, as early as 1900 Beard, working on the stingray, had proposed that the thymus is the principal source of ‘leucocytes’. This prescient suggestion – forgotten for the next 60 years – presaged the crucial finding of Miller in 1961 that removing the thymus from neonatal mice induced a wasting syndrome associated with impaired cellular immunity. Miller's conclusion that the thymus is the source of ‘immunologically competent cells’ and that ‘lymphocytes leaving the thymus are specially selected cells’ began the modern age of thymology.

Thymus

Thymus

First of 2 graphics depicting the anatomy of the normal thymus is shown. The thymus is a bilobed organ with a central isthmus composed of multiple lobules covered by a thin capsule. The thymus is located immediately anterior to the superior aspect of the pericardium and the origins of the aorta and pulmonary trunk.

The thymus and spleen

The thymus and spleen occur in all jawed vertebrates, appearing to have the same function in T cell development as in mice and humans. The thymus, therefore, likely gives rise to both αβ and γδ T cells in all jawed vertebrates. There are minor anatomical differences in thymus structure among different lineages. In most fish the two lobes of the thymus are separated on either side of the body cavity. In some marsupials—koalas for example—there is both a thoracic thymus and a cervical thymus, the latter being a second set of thymic lobes above the thoracic cavity. What function this second thymus serves remains unknown.

It has also been suggested that the thymus evolved from specialized GALT (Matsunaga and Rahman, 2001). The mammalian thymus is derived from the third pharyngeal pouch. The second pharyngeal pouch becomes GALT in the form of the palatine tonsil. The second pharyngeal pouch develops into thymus in some fish, whereas other species have six pharyngeal pouches, all of which develop into a thymus. The fish thymus is therefore in contact with the external environment and is anatomically linked to the gills. These data suggest that GALT developed first and that it was only subsequently that higher animals subordinated its activity from GALT into a primary lymphoid organ.

T lymphocytes (thymus-dependent)

Thymus-dependent (T) lymphocytes recognize and destroy foreign cells and regulate immune reactions. T lymphocytes carry out these functions directly by cell-to-cell contact or indirectly by using factors they produce and secrete. T cells regulate the activity of other T cells, macrophages, B cells, neutrophils, eosinophils, and basophils. T lymphocytes mature in the thymus because lymphoid cells differentiate in the thymus; they require specialized functions, and their cell membranes display distinguishing profiles or differentiation antigens. The biologic function of many of the differentiation antigens is not yet understood, but these cell surface markers are extremely useful for identifying lymphoid T cell subsets in normal and lymphoproliferative disease states. T lymphocytes, after leaving the thymus, are mainly found in the blood and thymus-dependent areas of the lymphoid tissues (e.g., spleen, lymph nodes, and Peyer's patches). In the circulation, they make up 80–90% of the total lymphocytes. The result of antigen recognition by B and T cells is similar but also different in fundamental ways. After antigenic stimulation, both cells become activated, proliferate, and differentiate into memory cells. The end of B cell activation is simply immunoglobulin secretion, whereas T cells have major functions as a result of antigenic stimulation:

they produce a series of non-immunocompetent but soluble mediators called cytokines; and

they may kill the cell containing the antigen.

With use of the differentiation antigens, human T cells have been characterized by monoclonal antibody techniques and are divided into three functional subgroups: helper, suppressor, and cytotoxic lymphocytes. Helper T lymphocytes, which constitute about one third of the mature cell population, are programmed to produce factors that amplify the functions of other cells (B lymphocytes and other T lymphocytes). Suppressor lymphocytes also have characteristic functions and can suppress effector lymphocytes by direct interaction or by the release of soluble suppressor factors. Cytotoxic T cells also express antigens, which can be detected by monoclonal antibodies.

T lymphocytes primarily recognize cell-associated antigens. Specifically, T cells simultaneously recognize both the antigen and a portion of one of the self class I or class II major histocompatibility complex (MHC) gene products. It is not clear whether T cells have distinct receptors for this MHC and the antigenic determinant or single receptors that have both specificities combined. Activated lymphocytes can produce a variety of soluble effector substances. They participate in the complicated process called the immune response. These substances are called cytokines and include, among others, those listed in Table 19-2.

T lymphocytes are required for several types of reactions, such as regulating immunoglobulin production, mediating delayed hypersensitivity, and lysing early virus-infected cells. An important advance in understanding how T cells develop into functionally mature populations and how they mediate these functions was the demonstration that distinct stages of T cell differentiation and function correlate with the expression of specific surface molecules. These molecules are designated by the letter T followed by a number (T1–T11) and can be detected by monoclonal antibodies.