胸腺退化和免疫衰老人和实验动物衰老时都毫无例外地发生胸腺退化。在人类,胸腺实质细胞明显退化是从性成熟开始,至45～50岁时退化完成。退化的胸腺内仅残留原来5～10％的细胞,胸腺退化是免疫系统形态和功能改变的原因。作为一个内分泌器官的胸腺能合成和分泌若干种多肽类激素,对淋巴细胞的分化起重要作用。人胸腺激素的血浓度在20～30岁前保持恒定,以后逐步下降,正常人在60岁后不再能测得。未成熟的淋巴细胞从骨髓进入胸腺皮质发育成熟,而老年人这种转运减 Thymus degeneration occurs without exception in both thymic degeneration and immunodeficiency and experimental animals. In humans, significant degeneration of thymus parenchyma cells starts from sexual maturity, and degenerates to 45 to 50 years of age. Only 5% to 10% of the original cells remain in the degenerated thymus, which is responsible for the changes in the morphology and function of the immune system. As an endocrine organ in the thymus can synthesize and secrete a number of peptide hormones, lymphocyte differentiation plays an important role. Thymus hormone concentrations in the blood of 20 to 30 years of age remained constant after the gradual decline in normal people can no longer be measured after the age of 60. Immature lymphocytes from the bone marrow into the thymic cortex maturation, while the elderly transporter less