The study of the interactions among behavioral, neural and endocrine, and immune functions. It is perhaps the most recent convergence of disciplines that has evolved to achieve a more complete understanding of adaptive processes. At one time, the immune system was considered an independent agency of defense that protected the organism against foreign material (that is, proteins that were not part of one's “self”). Indeed, the immune system is capable of considerable self-regulation. However, converging data from the behavioral and brain sciences indicate that the brain plays a critical role in the regulation or modulation of immunity. This research indicates that the nervous and immune systems, the two most complex systems that have evolved for the maintenance of homeostasis, represent an integrated mechanism for the adaptation of the individual and the species. Thus, psychoneuroimmunology emphasizes the study of the functional significance of the relationship between these systems—not in place of, but in addition to, the more traditional analysis of the mechanisms governing the functions within a single system—and the significance of these interactions for health and disease. See also: Neuroimmunology