Thymus and T cells of the Adaptive Immune System


Figure 1: Magnification of Hassall's corpuscle in human thymus.
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Figure 2: Immunohistochemical staining in human thymus showing protein expression of PSMB11, CD8B1 and THEMIS

Thymus is a gland, and one of the primary lymphoid organs where T cell maturation is taking place. T cells are the major component of the adaptive immune system.

The thymus is mostly active in the neonatal period and pre-adolescence, but after puberty it begins to atrophy and glandular tissue will be replaced by fat cells. This process is called involution, which means shrinking of an organ. Histologically it is located beneath the sternum in the rib cage and consists of two lobes with a cortex and a medulla. The lobes are further divided into lobules, separated by a septum (a wall). The cortex has a higher cell density compared to the medulla. In medulla, the Hassall's corpuscles are found (Figure 1). This structure consists of epithelial cells that are flattened and circular in shape - like a galaxy seen from far away. The corpuscles will increase in number as we age and have no secretory function.

The thymus is one of the primary lymphoid organs where T cell maturation is taking place. T cells are created in the bone marrow and thereafter transported to thymus for maturation. The T cells, divided into several subtypes; cytotoxic T cells, T helper cells, memory T cells, regulatory T cells and natural killer T cells, play an important role of the adaptive immune system. This blog post will look at couple of genes associated with T cells. Before the T cells can enter the bloodstream, they need to go through several steps related to cell maturation and their faith will be granted in the final steps of the maturation process. They could either be positively selected or negatively selected, where the negatively selected T cells are eliminated because of autoreactivity and the positively selected can enter the blood as mature T cells.

One important gene for T cell maturation is PSMB11 (Figure 2), which is present in the cortical regions of the thymus. This gene encodes a proteasome subunit. Proteasomes will generate peptides that are presented by major histocompatibility complex (MHC) to other immune cells.The gene CD8B1 (Figure 2) encodes a cell surface glycoprotein that can be found on cytotoxic T cells and is a mediator of cell-cell interactions within the immune system. This protein is involved in recognizing antigens provided by antigen presenting cells (APCs). The THEMIS (Figure 2) gene encodes a protein involved in the late phases of T cell development. It is necessary for lineage commitment and functions through T cell antigen receptor signaling.

A list of proteins analyzed with immunohistochemistry in thymus can be found here!

Emil Lindström