[Immunological deficiency syndromes, immunosuppression by drugs and occurrence of neoplasms: a casual association?].

Although supported by a number of experimental models, the assumption assigning a crucial role to the immune system in the antineoplastic defense mechanisms has not been convincingly demonstrated so far for human tumors. Should the theory be correct, severe functional impairment of the immune system would obviously result in the occurrence of tumors with abnormally high frequency. Registry holdings systematically collecting pertinent information on the malignancies developed in patients with primary immunodeficiency diseases or in organ transplant recipients maintained on therapeutically-induced immune depression, as well as the observation of tumors occurring in patients treated with immunosuppressive agents and of second malignancies arising after radio- and/or chemotherapy of the primary tumor consistently indicate that depressed immunity is usually associated with an increased incidence of cancer as compared with that expected in the general control populations. However, not all types of tumors are increased to the same extent, in that lymphoreticular neoplasias (especially non-Hodgkin's lymphomas), acute leukemias as second tumors and, among solid neoplasms, squamous cell carcinomas are those most frequently reported. These observations suggest that even deeply impaired tumoricidal immune mechanisms may facilitate the growth of certain tumors only, especially of those arising from the cells of the immune system itself, in remarkable contrast with their frequency in the general population. Oncogenesis may be favoured in various states of depressed immunity by a number of ways. Their elucidation might have bearing on the comprehension of the more general phenomenon of the neoplastic transformation.