Blood transfusion and immunomodulation: a possible mechanism

To induce an immunogenic response in vivo, an antigen-presenting (stimulator) cell must present both antigen-specific (class II MHC) and an accessory signal to the CD4 T cell. Failure to express the accessory signal has been shown in vitro to induce a state of specific unresponsiveness (anergy) in the T cell. We have shown that although stimulator cells in blood continue to express class II MHC molecules during refrigerated storage, their ability to present the accessory signal diminishes, reaching zero in all units tested by about 13 days. This implies that blood in excess of 2 weeks old should not alloimmunize but could induce some degree of immunosuppression. UV-B irradiation and, to a lesser extent, gamma-irradiation, were also shown to inhibit expression of the accessory signal by stimulator cells in blood.