For many years,
expression of MHC class I genes in unstimulated neurons of the
central nervous system (CNS) has been considered to be low or absent.
The lack of MHC class I antigens might account for the immune privileged
status of the CNS. Recent results, however, revealed MHC class
I expression in certain populations of unstimulated neurons. Further
examinations indicated that MHC class I expression closely parallels
the spatio-temporal pattern of activity-dependent plasticity in
the developing and adult mammalian brain and therefore may play
a role in neuronal signaling.