Immunological aspects of blood transfusions.

Almost all identified acute and/or severe immunological reactions towards blood transfusions, reported by surveillance systems such as SHOT (Severe Hazards of Transfusion) in the UK are mediated by allo-antibodies. In contrast, the clinical effects of transfusion-induced cellular immunity are virtually unknown. Although alterations in lymphocyte responses and natural killer cell functions after blood transfusion has been reported in many publications, the relevance of these in vitro assays for in vivo immunity are lacking. Even for clinically obvious immunomodulatory effect of blood transfusions, such as the mitigation of renal graft rejection, no uniform in vitro explanation has been identified. In the laboratory animal it has been shown that when two antigenic stimuli are given simultaneously, the response to one of these antigens is often decreased. Blood transfusions introduce a multitude of foreign antigens. Indeed, immunostimulation and suppression by blood transfusions have both been found. Systematic studies on immunological side-effects of blood transfusions are hardly available. Since the UK and France introduced a transfusion vigilance system, severe immunological side-effects such as haemolytic reactions, TRALI (acute lung injury), PTP (post-transfusion purpura) and graft vs. host disease are registrated in these countries and their incidence can be estimated based on the national number of transfusions. However, every blood transfusion interferes with the immune system of the recipient. The available evidence of harm from immune responses not leading to severe transfusion reactions will be discussed.