Allogeneic blood transfusion is the most frequent allotransplantation
procedure performed on a routine basis with no prior HLA-typing. Rough
ly 50% of the recipients of unprocessed red cells and platelets become
alloimmunized. Evidence also exists for some degree of transfusion-in
duced immunosuppression. Prior transfusion has been shown to enhance k
idney transplant survival and evidence of an increase in tumor recurre
nce and of infectious complications has also been presented. The prese
nce of donor antigen-presenting cells appears to be a prerequisite for
alloimmunization and they must be both viable and capable of presenti
ng a costimulatory signal in order to induce IL-2 secretion and prolif
eration of responding CD4 T cells. APCs presenting antigen but no cost
imulatory signal can induce non-responsiveness in CD4 T cells, a possi
ble mechanism of transfusion-induced immunosuppression. APCs in refrig
erated blood continue to present antigen but progressively lose their
ability to provide costimulation. By day 14 costimulatory capacity is
absent and transfusion of such blood should not alloimmunize but could
induce some degree of immunosuppression. Further refrigerated storage
in excess of 2 to 3 weeks leads to induction of apoptosis in contamin
ating leukocytes. We have found that alloantigens expressed on such ca
lls do not appear to be recognized by responder T cells and transfusio
n of blood stored in excess of 3 weeks should neither alloimmunize nor
immunosuppress.