[Why remove leukocytes from labile blood products in 1995?]

During the last 15 years, the techniques to prepare leukocyte-poor cellular blood components greatly improved, as well as our knowledge about the role of leukocytes in many adverse effects of transfusion. These two facts favor the extension of indication of leukocyte-poor blood components. Leukocytes in blood components may be detrimental to their storage, due to their metabolic needs and to their progressive lysis, leading to the release of cytokines. Leukocytes are the exclusive vector in blood of CMV and HTLV viruses. Leukocytes are a key element of the immune modifications induced by transfusion. HLA alloimmunization is favored by the transfusion of large quantities of leukocytes HLA different from the recipient whose immune functions are intact. Conversely, the risk of transfusion associated graft versus host disease is dependent of the transfusion of mature T lymphocytes sharing a partial identity with the recipient, and/or an immune deficient status of the recipient. Between these two extremes, many other effects related to the presence of leukocytes in cellular blood components, as are the transfusion effect observed in transplant recipients, the increased risk for bacterial infection after transfusion, the increased risk for tumor recurrence or the reactivation of virus infections, remain to be fully understood. Despite recent significant improvements, further studies, experimental as well as clinical, will be needed to expand the indications of leukocyte-poor blood components.