Host defense mechanisms in protein energy malnutrition.

This review has focused on effects of protein energy malnutrition on immune responses in the human host. These studies document major impairment of the T-cell and complement systems in severe PEM, and less profound, but probably significant, effects upon B-cells and immunoglobulins, particularly SIgA. While mild-moderate malnutrition also alters the T-cell system and may predispose to infection, there is less evidence to suggest that complement is similarly affected. Indeed, some data suggest that the host with mild to moderate malnutrition is still able to respond to stress with an acute phase serum protein response and to boost serum levels of complement and complement activity. This may be a functionally significant distinction, serving to separate the more from the less severely ill. Because many other factors alter immune responses, including vitamins, calories, and trace metals, and few clinical studies have examined these parameters, it is uncertain how much of the problem in malnutrition is due to protein, to energy intake, to iron, to other micronutrients and trace minerals, to vitamin E or to other vitamins alone or in combination with deficiencies in protein and energy. Other chapters in this volume attempt to sort out these questions in animal studies, but the relevance of these data for the human situation will remain uncertain until the investigations are carried out in humans as well.