Although the relationship between starvation and reduced resistance to infection has been suggested by historical accounts of famines and pestilence and by recent epidemiologic studies, the concept of nutritional deficiency causing impairment of immunocompetence is relatively recent. In PEM (protein-energy malnutrition), most of the host defence mechanisms are breached, especially manifested in reduced cell-mediated immunity. One plausible reason is the reduction in mature fully differentiated T lymphocytes. Recent immunological methods made it possible to analyze its precise mechanism and process in detail. Naturally occurring states of malnutrition are difficult to interpret largely because deficiencies usually involve multiple dietary factors. It is obvious that single nutrients can only be analyzed in defined and controlled animal experiments. Role of micronutrients such as zinc, iron, and vitamins, specific amino acids such as arginine, glutamine, and fat on immunological responses have been paid attention on their specific actions and some of them have been investigated in humans as well.