This variability actually decreases in anticipation of a mental task, as sympathetic stimulation increases. Cortisol is a catabolic hormone that stimulates tissue breakdown. For example, cortisol levels are elevated in starvation situations when muscle tissue is catabolized for energy. The immune system is a simple term for an intrinsic, adaptable cellular system that responds to invading foreign substances and eliminates them, or minimizes their capacity to replicate. The immune sytem is modifiable both in the rapidity and magnitude of its response to a foreign invader.
Exercise creates both acute and chronic changes in immune system function.
Acute exercise has been shown to cause a transient depression of certain components of the immune system, creating a window of susceptibility to infection of several hours after an exercise bout. The stress of chronic exercise has a biphasiceffect on immune function. This can be demonstrated several ways. Second, the magnitude of immune response to an applied antigen is decreased in overstressed athletes. Diagnostic kits are available which allow the controlled application of 7 allergens to the skin of the forearm.