**7. Conclusions**

Transfusion of allogeneic blood components is commonly used in cardiac surgery. Several observational and randomized studies found higher morbidity and mortality if patients were transfused with allogeneic blood products. In the last years the clinical effects of transfusion triggers, the storage times and the presence of allogeneic leukocytes in red blood cells were investigated intensively. In cardiac surgery it has been found that allogeneic blood transfusions could increase postoperative complications, which is controversial in the last years. Clinical effects of storage of RBCs are discussed intensively and until now there is no clear evidence that older RBCs are deleterious in cardiac surgery. It has been found that mainly leukocyte-containing blood products play a crucial role in the development of postoperative complications in cardiac surgery. To understand the differences between leukocyte-containing and leukocyte-depleted RBC transfusions we described in this review several possible causal mechanisms. Soluble mediators derived from deteriorating leukocytes during storage of RBC are unlikely to play a role. The complement activation by lectin pathway may be relevant to explore as a causal deleterious effect of plasma transfusions, although does not explain excess death by MODS in association with allogeneic leukocytes. An acute phase reaction represented by procalcitonin could be excluded as a mediator induced by allogeneic leukocytes. A difference in cytokine responses in the recipient was the only significant factor that could be identified as playing a possible causal role. In most countries of the Western World transfusion of leukodepleted blood components is standard practice. Although the final conclusion on this issue is not made yet. Furthermore other factors, such as plasma and platelet transfusions (due to activation or storage lesions) and the (possible) activation of the coagulation system by the allogeneic blood transfusions, may remain to play important roles in the development of transfusionassociated complications and are input for further improvement of transfusion management in cardiac surgery. Thus many residual questions have still to be answered in the future.
