[Immunodeficiency and cancer].

The analysis of cancers associated with immunodeficiencies (either congenital, iatrogenic or acquired) enabled us to define the possible role of these deficiencies in the sequence of events leading to cancer and in particular the emergence of hematopoietic proliferations. Immunodeficiency increases the risk of viral infection, and in severe deficiencies, uncontrolled infections (Epstein-Barr virus) are directly responsible for the appearance of polyclonal proliferations; in moderately severe deficiencies, the hyperstimulation of the immune system by repeated infections increases the number of mutagenic events, but the immortalization of a malignant clone is a late event, which may or may not be linked to a viral infection. On the other hand, some intrinsic congenital or acquired abnormalities of lymphoid cells may be responsible both for immunodeficiency and carcinogenesis (chromosomic fragility). Apart from any immunological reaction, the immune system plays a role in the proliferation and differentiation events in various tissues; and any dysregulation of the latter may indicate the emergence of a cancer.