The lymphatic tissues, organs, and physiological processes that identify an antigen as abnormal or foreign and prevent it from harming the body. The body is protected from pathogen invasion by the skin, mucosa, and normal flora of the gastrointestinal tract and skin; chemicals contained in tears; the sebaceous gland s; gastric acid; and pancreatic enzymes. The bone marrow produces white blood cells (WBCs), the body's primary internal defense. Lymphoid tissues, including the thymus gland , spleen, and lymph nodes, influence the growth, maturation, and activation of WBCs; lymphoid tissue in the gastrointestinal and respiratory tracts and mucous membranes contain WBCs for site-specific protection. Finally, physiologically active protein mediators, called cytokines, help regulate the growth and function of immunologically active cells.

Effects of stress: Investigations of the influence of stress on susceptibility to disease have shown that in some but not all people who suffered stressful events, the possibility of onset of illness increased. A decrease in the usual number of pleasant events was a stronger predictor of susceptibility to illness than was an increase in unpleasant ones. Negative experiences included criticism, frustration, irritating encounters with fellow workers, deadlines, heavy workload, and burdensome or unpleasant chores or errand s. Even though the concept that stress lowers resistance to disease appears to apply only to some people, the explanation of this mechanism has not been established.