[Lymphocytopenia: aetiology and diagnosis, when to think about idiopathic CD4(+) lymphocytopenia?].

Lymphocytopenia is defined by a lymphocyte count less than 1500/mm(3) in adults and less than 4500/mm(3) in children before the age of 8 months. Lymphocytopenia can be global or selectively affect a peculiar lymphocyte subpopulation. The patient's age, the context as well as the associated clinical manifestations and treatment prescribed must be taken into account in order to identify the etiology of lymphocytopenia. In adults, lymphocytopenia can be caused by: (1) insufficient thymic output (primary immune deficiencies, corticosteroid treatment, zinc deficiency, etc.), (2) increased lymphocyte catabolism (radiotherapy, chemotherapy, immunosuppressant, HIV infection, systemic lupus, etc.), (3) modified lymphocyte distribution (viral infections, septic shock, extensive burns, splenomegaly, granulomatosis, etc.), (4) multifactorial or unknown etiology (end-stage renal disease, lymphoid malignancies, solid tumor, ethnicity, etc.). In children, in addition to these etiologies, other immune deficiencies may be responsible for severe lymphocytopenia (thymocytes apoptosis, cytokine deficiencies, altered B-cell and T-cell receptor synthesis, signal transduction and cellular interactions deficiencies). Idiopathic CD4(+) lymphocytopenia is a rare disorder. It is defined by a persisting lymphocyte CD4(+) count less or equal to 300/mm(3) or less or equal to 20% of total lymphocytes in the absence of alternative diagnosis. Clinical symptoms can be absent or include opportunistic infections, auto-immune manifestations, lymphoma or solid tumors. Treatment is similar to that of HIV-infected patients and sometimes relies on specific immunotherapy even though clinical benefit has not been evaluated.