Regarding NK cells the story has become quite interesting in the past 3
years. That is because reports have determined that NK cells do have
receptors and they do recognize MHC. The difference is that recognition
is a negative event. they get turned off not on. Also, they aint just
killers anymore, these cells secrete cytokines, and may also play a role
in tissue remodeling via metaloproteinase secretion. Moretta's lab in
italy and Seamark here in the states would have some info. Also,
G. Trinchieri and R. Herberman have written extensive reviewson these
cells.