Abstract Malnutrition and respiratory disease commonly occur together, and the immunologic defects caused by malnutrition may affect clinical responses and the impact of vaccines. These defects include impaired maturation and function of T lymphocytes, diminished secretory immune responses, poor humoral antibody responses to polysaccharide antigens, deficient complement activity (especially the alternative pathway), and impaired antimicrobial mechanisms by phagocytic cells. In addition, protein-energy malnutrition often leads to very high serum levels of IgE as a result of altered T cell regulation ofIgE production and may contribute to IgE-mediated pathologic responses to respiratory syncytial virus or parainfluenza virus infection. Correction of these deficits may improve host response to infection and enhance vaccine-related protection. Both vitamin A and iron deficiency states have been epidemiologically associated with increased morbidity due to respiratory infection. The impact of preventive therapy with either vitamin A or iron is still uncertain, but such therapy has the potential for diminishing the incidence of respiratory infections and their consequences. This content is only available as a PDF. © 1991 by The University of Chicago