Acute and Stress-Induced Involution of the Thymus

Age-related, physiologic involution of the thymus, which commences in puberty, stands in juxtaposition to acute, stress-related involution, which is of diagnostic and therapeutic significance especially during childhood and adolescence. The latter represents an endogenous (e.g. illness-related) or exogenous (e.g. drug-related) very rapid acceleration of physiologic, age-related involution, and is brought about by endogenous steroid release. This release induces lysis of lymphocytes and blocks their mitosis, resulting in a rapid reduction in the weight and size of the thymus which can be seen radiologically only a few days after the onset of the stress situation (e.g., after the beginning of an illness). Just as stress situations lead to atrophy of the thymus, so their termination (e.g., in convalescence or after exogenous induction) allows renewed growth of the thymus in the form of so-called regenerative hyperplasia (rebound phenomenon) (see Sect. 7.4.6).