The thymus is a bilobed organ positioned in front of the aorta and behind the manubrium of the sternum (fig. 11.33). Although the size of the thymus varies considerably from person to person, it is relatively large in newborns and children, and sharply regresses in size after puberty. Besides decreasing in size, the thymus of adults becomes infiltrated with strands of fibrous and fatty connective tissue.
The thymus is the site of production of T cells (thymus-dependent cells), which are the lymphocytes involved in cellmediated immunity (see chapter 15). In addition to providing T cells, the thymus secretes a number of hormones that are believed to stimulate T cells after they leave the thymus.
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