Immunology | Immune system| Immunity

               The Thymus is the site of T-cell and maturation.It is a flat,bilobed organ situated above the heart.Each lobe is surrounded by a capsule and is divided into lobules,which are separated from each other by strands of connective tissue called trabaculae.Each lobule is organized into two compartment,or cortex,is densely packed with immature T cells,called thymocytes,whereas the inner compartment,or medulla,is sparsely populated with thymocytes.

Diagrammatic cross section of portion of thymus.

               Both the cortex and medulla of the thymus are criss-crossed by a three-dimensional stromal-cell network composed of epithelial cells,dendritic cells, and macrophages,which make up the framework of the organ and contribute to the growth and maturation of the thymocytes.Many of these stromal cells interact physically with the developing thymocytes.Some thymic epithelial cells in the outer cortex,called nurse cells,have long membrane extension that surround as many as 50 thymocytes,forming large multicellular complexes.Other cortical epithelial cells have long interconnecting cytoplasmic extensions that form a network and have been shown to intreract with numerous thymocyets  as they traverse the cortex.
               The function of the thymus is to generate and select a repertoire of T cells that will protect the body from infection.As thymocytes develop,an enormous diversity of T cell recptors is generated by a random process that produce some T cells with receptors capable  of recognizing antigen-MHC complexes.However,most of the T-cell-receptors produced by this random process are incapable recognizing antigen-MHC complexes and a small portion react with combinations of self antigen-MHC complexes.The thymus induces the death ot those that react with self-antigen-MHC and pose a danger of causing autoimmune disease.

Changes in Thymus with age.

            Aging is accompanied by a decline in thymic function.This decline may play some role in the decline in immune function during aging in humans and mice.The thymus reaches its maximal size at puberty and then atrophies,with a significant decrease in both cortical and medullary cells and an increase in the total fat content of the organ.Whereas the average weight of the thymus is 70 g in infants.its age dependent involution leaves an organ with an average weight of only 3 g in the elderly.