Mg participates in immune responses in numerous ways: as a cofactor for immunoglobulin synthesis, C'3 convertase, immune cell adherence, antibody-dependent cytolysis, IgM lymphocyte binding, macrophage response to lymphokines, T helper-B cell adherence, binding of substance P to lymphoblasts and antigen binding to macrophage RNA. Mg deficiency in rodents impairs IgG synthesis and cell-mediated immunity; complications include thymus atrophy, elevated IgE, hypereosinophilia, histaminosis and lymphoma. Immunologic sequelae of Mg deficiency in humans are subtle and may be affected by genetic control of blood cell Mg concentration. Abnormal C' activation, excess antibody production and susceptibility to allergy and to chronic fungal and viral infections have been reported. Mg appears to play a protective role in acute allergic reactions.