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Abstract
Numerous studies have demonstrated that a variety of hormones have receptors and exert
biologic actions on tissues of the immune system. Conversely, cytokines exert biologic
actions on the endocrine system. This bidirectional interaction is likely involved
in maintenance of physiological and immunologic homeostasis. This paper summarizes
a variety of actions of growth hormone (GH), prolactin (PRL), insulin-like growth
factor-I (IGF-I), glucocorticoids and thyroid hormones (TH) on the immune system.
It then proceeds to put these actions into a hypothetical context whereby these hormones
may mediate some changes in immune system function in response to environmental stimuli
such as physical and emotional stress, nutritional deprivation and environmental temperature.
In the first example, it is proposed that PRL secretion in response to stress may
serve an immunomodulatory role in two ways. The first is by stimulating the immune
system directly and the second is by dampening or reducing the degree to which glucocorticoids
are secreted in response to stress. The second example suggests that the increase
in GH secretion and reduced IGF-I secretion in response to protein/energy restriction
may have two potential immunomodulatory actions. One action is a direct effect of
GH on several components of the immune system. The other is the partitioning of nutrient
use away from skeletal muscle growth and toward tissues of higher priority such as
the immune system. The third example proposes that the increased secretion of TH during
cold environmental temperatures not only increases basic metabolic rate, but also
directly stimulates both primary and secondary lymphoid tissues. It is suggested,
therefore, that these three hormones are involved in maintaining immune system homeostasis
in response to environmental change.