In spite of discovery of new antibiotics and regular progress in intensive care, mortality from severe bacterial sepsis remains high. In this review the importance of cellular and humoral immunity in the pathogenesis and the outcome of severe infection is delineated. Immunological evaluation of patients in Intensive Care Units should be performed almost routinely in order to detect "high risk" patients with acquired defect in host-defence mechanisms. For these patients in addition to nutritional care, passive or active immunotherapy will help to restore resistance to bacterial infection.