HEALTH AND STRESS
Part VI begins with a description in Chapter 17 of the effects of both acute and long-term exercise on natural killer (NK) cells, neutrophils, macrophage/monocytes, and the lesser effects on T- and B-lymphocytes. In response to acute exercise, a rapid interchange of immune cells between lymphoid tissues and the circulation occurs. While the response depends on many factors, NK cells, neutrophils, and macrophages appear to be most responsive, both in terms of numbers and function. The only consistent finding to date with long-term exercise training is a significant increase in NK cell activity. Work performance tends to diminish with most systemic infections, and data suggest that increased severity of infection, relapse, and myocarditis may result when patients exercise vigorously.
Chapter 18 discusses the neuroendocrine consequences of systemic infection, emphasizing the primary role of the myriad polypeptide cytokines released into the circulation by lymphocytes, monocytes, macrophages, and endothelial cells. These pluripotent mediators induce the pathophysiological response termed the acute phase reaction, which is characterized by fever; nutrient catabolism; changes in protein, carbohydrate, and lipid metabolism; and profound changes in hepatic functions and in all components of the endocrine system.
Recent advances in identifying immune-neuroendocrine interactions are discussed in Chapter 19. The numerous interactions that have been characterized illustrate the important bidirectional communication between these two
systems. Perhaps the most important interaction relates to immune activation of the hypothalamic-pituitary-adrenal axis via cytokines. Stimulation of this counter-regulatory response plays a critical role in preventing the host from mounting an excessive defense response against "inflammatory stress". Because of these interactions, regulatory relationships exist whereby behavioral stimuli and inflammatory stress can ultimately modulate the function of the immune system.
The influence of biological rhythms on the immune system is presented in Chapter 20. Biologic rhythmic behavior has been characterized in levels of circulating white blood cells and subsets of these cells, cytokines and their inhibitors, and in the humoral immune response, although circadian time dependence of human responses to vaccination is less well documented. Some of the rhythms affecting the immune system are genetically fixed, but in certain frequencies (e.g., circadian) the rhythm may be adjusted in its timing by periodic environmental factors such as light-darkness, activity-rest pattern, temperature, and for some parameters, the timing of food intake. Many of these rhythms have potential major significance to the military.
Finally, Chapter 21 provides a summary of the workshop presentations, identifying those issues of importance to the military and identifying the issues that the CMNR would consider in their report.