[Omenn syndrome]

Omenn's syndrome is a rare autosomal recessive disease characterized by the onset, from the first weeks of life, of an exsudative skin rash, alopecia, hepatosplenomegaly, diffuse lymph-nodes, diarrhea and increased susceptibility to infections with hypereosinophilia. The associated severe combined immunodeficiency (SCID) differs from the other SCID by the existence of lymphocytosis. The number of T lymphocytes is normal or elevated; they are sometimes immature; the distribution of their subsets (CD4/CD8) is variable. The B lymphocytes are quantitatively and functionally deficient. Pathological investigations reveal major lymphoid depletion with severe thymic hypoplasia associated with a proliferation of cells which have the whole immunohistochemical characteristics of Langerhans' cells but do not contain their specific granulations (Birbeck's granulations) on the ultrastructural examination. These cells are also found in the skin, lungs and liver. The outcome is usually fatal before 1 year of age. The pathogenesis of this disease is still discussed: it might be the result, in an immunodeficient child, either of a graft-versus-host reaction after materno-fetal transfusion of immunocompetent cells, or of an abnormal immunologic reaction after antigenic stimulation, or a deficit of molecules in lymphocyte ecotaxy.