The relationship between nutrition and the immune system is complex since it is difficult to separate the effects of an illness from the immunodeficiency state. Often a vicious circle operates in the acute severely ill, in whom nutritional deficiency will impair the immune response, reducing protection against infection. The resultant infection can itself subsequently precipitate malnutrition. In general, severe protein deficiency has the most profound effect on cell-mediated immunity and a less pronounced effect on humoral immunity.

Cell-mediated immunity

In humans and animals severe protein deficiency impairs lymphocyte transformation in response to agents that stimulate T-cell proliferation in vitro, and the production of interferon by stimulated lymphocytes is reduced. Cutaneous delayed hypersensitivity reactions are consistently impaired in hospital patients with malnutrition. Although this has been used as a marker of protein energy malnutrition, suppression of skin tests is not specific as it occurs in other types of illness including acute infections. In general, the abnormalities of cell-mediated immune function patients are reversible with refeeding. Skin test responsiveness improves in malnourished children who receive a normal diet and circulating levels of lymphocyte subtypes rapidly return to normal.

Humoral immunity

Humoral immunity is less sensitive than cellular immunity to the effects of protein energy malnutrition. However, despite a normal or increased serum immunoglobulin concentration, the immunoglobulin turnover in response to an antigenic challenge may be impaired. Similarly, levels of secretory IgA in many external secretions are reduced in undernourished individuals despite a normal serum antibody response. These significant humoral abnormalities may contribute to the development of mucosal infections in malnourished patients.

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