Interactions of nutrition, infection and immune response. Immunocompetence in nutritional deficiency, methodological considerations and intervention strategies.

Clinical and epidemiologic data point to a causal interrelationship between nutritional deficiency and infectious illness. Both are major contributors to childhood morbidity and mortality, particularly in underprivileged population groups. Energy-protein undernutrition and deficiencies of iron, folates and pyridoxine, depress a variety of immunity functions. Delayed hypersensitivity and number of T lymphocytes are consistently reduced. In small-for-gestation low birth weight infants, cell-mediated immunity may remain depressed for several years. B lymphocytes, immunoglobulin levels and antibody responses are generally normal, but secretory IgA-antibody is reduced. Serum complement components are low and there is evidence of in vivo consumption of complement C 3. Neutrophil phagocytosis of bacteria and fungi is intact but the next step of intracellular killing is impaired. There are changes also in the production of lysozyme and interferon. Infection per se results in nutrient losses, either actual or by sequestration, and produces immunosuppression. The correction of postnatal nutritional deficits and/or infection is associated with reversal of immunological functions to normal. The interplay of nutrition, immunity and infection, and its biological implications are described.