In this review of progress in immunology, an attempt is made to synthesize some of the rapidly accumulating observations in clinical medicine and experimental biology into a workable scheme, which may help pediatricians in their approach to the study and management of patients suspected of abnormalities in their resistance to infection. Like all hypothetical schemes, based on incomplete knowledge and imperfect interpretation, it must be considered tentative and useful mainly in establishing hypotheses to be demolished by future investigation. However, it may clarify the bewildering torrent of case reports, experimental observations and speculations which are pouring out in the current immunological and clinical literature. The roles of the phagocytic system, the thymus, the lymphocytes, and the plasma cells with their secretory products, the immunoglobulins, in defense against infection, as well as the clinical manifestations arising from deficiencies of each of these elements in the immunological system of the body, are discussed. © 1968 The C. V. Mosby Company.