histomorphology of lymphoid tissues, delayed hypersensitivity skin reactions, lymphocyte antibody production, and complement activity. Subsequently, these observations have been extended by experimentation in laboratory animals and in work on human subjects. It is now recognized that nutritional deficiencies are associated with impaired cell-mediated immunity; reduced number of circulating T-lymphocytes, particularly CD4+ helper T-cells and CD3+ CD25+ T-cells that bear the interleukin (IL)-2 receptor; decreased lymphocyte stimulation response to mitogens and antigens; altered production of cytokines; lower secretory IgA antibody response on mucosal surfaces; decreased antibody affinity; and phagocyte dysfunction. Similar alterations in immune responses have been reported with deficiencies of individual nutrients, such as protein, essential fatty acids, vitamin A, vitamin E, pyridoxine, folic acid, zinc, iron, copper, and selenium (Chandra, 1991, 1992a; Meydani and Hayek, 1992).

Today, nutritional immunology forms the basis of semester-long graduate courses, week-long symposia, and expansive monographs. Thus, to provide a complete answer to the question "What do we know about nutrition and immune responses?" is not easy or simple. Instead, a selective review and some recent observations are provided below.

Several general principles and conclusions on nutrition and immunity can be stated:

• Protein-calorie malnutrition and deficiencies of individual nutrients, even subclinical deficits, are associated with impaired immune responses and altered risk of infection.
• Excessive intake of some nutrients may result in reduced immune responses.
• Dose-response curves should form the basis of recommendations for optimal nutrient intake.
• Immune responses are sensitive and functional indices of nutritional status and can aid in assessing prognosis in medical and surgical patients.
• Several factors other than nutrition can modulate immunocompetence.
• Basic knowledge of nutrition and immune interactions can be utilized to formulate nutritional recommendations and interventions that may reduce illness and improve chances of survival.