Leucodepletion and immune response mechanisms.

The recipient and donor mechanisms responsible for mediating alloimmunity and immunomodulation during blood transfusions are complex and most probably superimposed thus creating multilevel fields of potential interaction and regulation. For example, at the recipient level, both the innate and adaptive immune systems can be significantly affected by various types of transfusions [1,2]. In addition, the health status of the recipient plays a critical role in determining how the host will immunologically respond to transfusion. On the other hand, donor transfusion products have many characteristics that can potentially influence recipient immunity and its regulation (e.g. age, leucocyte content, etc.). Understanding how these varied parameters culminate in an immune response or not is fundamental to developing safer blood products and better care for recipients. Many prospective randomized studies have shown that leucoreduction of blood components can reduce the incidence of alloimmunization and platelet refractoriness [3]. Mainly based on these data, leucoreduced components are now recommended for any patient requiring long-term transfusion support such as patients with leukaemia, lymphoma, myelodysplasia or those undergoing stem cell transplantation. Although leucoreduction has shown a clear benefit by reducing the incidence of alloimmunization and refractoriness in these immunocompromised patient groups, it is not clear how leucoreduced blood products may affect the immune response in immunocompetent individuals such as trauma or surgical patients. However, in a recent randomized controlled study with 404 cardiac surgery patients, a single multiunit transfusion of either leucofiltered RBC (stored or not) or buffy coat-reduced RBC induced similar levels of anti-HLA alloimmunization [4]. These results suggest that not all patient groups may benefit from leucoreduced blood products and other treatment modalities may need to be developed to reduce alloimmunization in those patients. Understanding the immune response against leucoreduced blood products in healthy recipients can be accomplished by studying animal models of transfusion. This paper will focus on how, in healthy recipients, leukoreduction may have a ‘double-edged sword’ effect on alloimmune mechanisms.