In young children, the thymus extends into the neck and the chest, but after puberty, it begins to shrink. The size of the thymus in most adults is very small. Like some other endocrine glands, the thymus has two lobes connected by a stalk. The thymus secretes several hormones that promote the maturation of different cells of the immune system in young children. In addition, the thymus oversees the development and "education" of a particular type of immune system cell called a T lymphocyte, or T cell.
Although many details of thymal hormonal activity are not clear, at least four thymal products have been identified which control T cell and B cell (antibody-producing immune cells) maturation. The four products are: thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin. Because the viral disease, AIDS, is characterized by T cell depletion, some AIDS treatment approaches have tried administering tymosin to boost T cell production.
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