Immunologic aspects of blood transfusion.

For 20 years, clinical reports and laboratory observations have suggested that allogeneic blood transfusion effects important changes in the recipient's immune response. The seminal clinical studies involved dose-dependent improvement in renal allograft survival in patients transfused with allogeneic whole blood, red blood cells, and buffy coat preparations. Subsequently, a burgeoning, but unclear literature proposed that allogeneic blood transfusion decreases survival or tumor-free survival of patients who undergo operations for a variety of different malignancies. Similar studies suggest that the risk of postoperative infection increases when patients receive allogeneic blood. Transfusion reportedly improves some patients with Crohn's disease. In summary, these findings have been interpreted as evidence for an immunosuppressive effect of allogeneic blood transfusion. A small prospective study showed that paternal buffy coat infusion decreases the rate of fetal loss in a subset of women with recurrent abortion. These data suggest induction of "tolerance." Laboratory studies confirm changes in lymphocyte subsets, lymphocyte activation, natural killer cell activity, antigen-presenting function, and phagocytic cell function in patients and animals that receive allogeneic blood. The clinical relevance of these observations remains controversial. Allogeneic leukocytes induce expression of latent cell-associated viruses (human immunodeficiency virus, cytomegalovirus, Epstein-Barr virus), suggesting further immune-mediated adverse effects of transfusion. The mechanisms and clinical importance of these observations have become areas of intense interest and investigation for transfusion medicine.