To induce an immunogenic response in vivo, an antigen-presenting (stim
ulator) cell must present both antigen-specific (class II MHC) and an
accessory signal to the CD4 T cell. Failure to express the accessory s
ignal has been shown in vitro to induce a state of specific unresponsi
veness (anergy) in the T cell. We have shown that although stimulator
cells in blood continue to express class II MHC molecules during refri
gerated storage, their ability to present the accessory signal diminis
hes, 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 le
sser extent, gamma-irradiation, were also shown to inhibit expression
of the accessory signal by stimulator cells in blood.