<P> The immune system is involved in many aspects of physiological regulation in the body . The immune system interacts intimately with other systems, such as the endocrine and the nervous systems . The immune system also plays a crucial role in embryogenesis (development of the embryo), as well as in tissue repair and regeneration . </P> <P> Hormones can act as immunomodulators, altering the sensitivity of the immune system . For example, female sex hormones are known immunostimulators of both adaptive and innate immune responses . Some autoimmune diseases such as lupus erythematosus strike women preferentially, and their onset often coincides with puberty . By contrast, male sex hormones such as testosterone seem to be immunosuppressive . Other hormones appear to regulate the immune system as well, most notably prolactin, growth hormone and vitamin D . </P> <P> When a T - cell encounters a foreign pathogen, it extends a vitamin D receptor . This is essentially a signaling device that allows the T - cell to bind to the active form of vitamin D, the steroid hormone calcitriol . T - cells have a symbiotic relationship with vitamin D. Not only does the T - cell extend a vitamin D receptor, in essence asking to bind to the steroid hormone version of vitamin D, calcitriol, but the T - cell expresses the gene CYP27B1, which is the gene responsible for converting the pre-hormone version of vitamin D, calcidiol into the steroid hormone version, calcitriol . Only after binding to calcitriol can T - cells perform their intended function . Other immune system cells that are known to express CYP27B1 and thus activate vitamin D calcidiol, are dendritic cells, keratinocytes and macrophages . </P> <P> It is conjectured that a progressive decline in hormone levels with age is partially responsible for weakened immune responses in aging individuals . Conversely, some hormones are regulated by the immune system, notably thyroid hormone activity . The age - related decline in immune function is also related to decreasing vitamin D levels in the elderly . As people age, two things happen that negatively affect their vitamin D levels . First, they stay indoors more due to decreased activity levels . This means that they get less sun and therefore produce less cholecalciferol via UVB radiation . Second, as a person ages the skin becomes less adept at producing vitamin D . </P>

Where is the immune system found in the human body