Humoral immunity in malnutrition.

Malnutrition affects the humoral immune system in diverse fashions. B lymphocyte subpopulations, serum IgG, and IgA levels, and immunoglobulin synthesis and metabolism are usually normal or increased. Hypogammaglobulinemia may occur in very young, severely malnourished infant. Although usually normal, deficient antibody responses to injected antigens are occasionally present, depending on the severity of the malnutrition. Serum IgE levels are usually high in malnutrition, probably due both to the increased incidence of intestinal parasites and to the lack of T cell control of IgE. Despite these high IgE levels, allergy is unusual. Secretory IgA levels in the respiratory and gastrointestinal (GI) fluids are generally decreased, as are secretory IgA antibody responses. This local antibody deficiency may increase GI permeability, stimulate immune complex formation, and result in the formation of IgG food antibodies. Breast milk secretory IgA levels are equivalent in malnourished mothers but because of lessened milk volume, the total amount of secretory IgA delivered to their offspring is diminished. Complement levels are somewhat diminished in the presence of malnutrition with a resultant opsonic deficiency. Although these B cell system aberrations undoubtedly contribute to the enhanced susceptibility to infection of malnourished patients, they are usually less severe than are concomitant T cell deficiencies.