Severe acute respiratory syndrome (SARS) is a new human infectious disease. The causative agent of SARS is a novel coronavirus (SARS-CoV). This report summarizes the hematological findings in SARS patients and proposes the possible mechanisms of SARS-CoV related abnormal hematopoiesis. Hematological changes in patients with SARS are common and include lymphopenia, thrombocytopenia and occasionally leukopenia. A significant decrease was also observed in peripheral CD4+ and CD8+ T lymphocyte subsets and it was related to onset of SARS. A number of potential mechanisms may be involved. The development of auto-immune antibodies or immune complexes triggered by viral infection may play a major role in inducing lymphopenia and thrombocytopenia. Moreover, SARS-CoV may also directly infect hematopoietic stem/progenitor cells via CD13 or CD66a inducing their growth inhibition and apoptosis. The receptor for group I and III CoV is aminopeptidase N (CD13). CD13 has been identified in human bone marrow CD34+ cells, platelets, megakaryocytes, myeloid cells, and erythroid cells, but not in lymphocytes. The common receptor for group II CoV is CEACAM1a (CD66a). CD66a is an adhesion molecule expressed on bone marrow CD34+ cells, platelets, granulocytes and activated lymphocytes. In addition, glucocorticoids could induce lymphopenia and the use of steroids may account for the decrease of lymphocytes in some SARS patients. The increased consumption of platelets and/or the decreased production of platelets in the damaged lungs are a potential alternative but often overlooked mechanism that can contribute to thrombocytopenia in severe critical pulmonary conditions.

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