tokens: Dilutions of H - 2b or H - 2d NP peptides indicated that 3 - 4 logs less H - 2b NP peptide was required to sensitize syngeneic target cells for CTL - specific lysis , suggesting that the differing affinities of H - 2b and H - 2d major histocompatibility complex molecules for their peptides likely account for the total removal of NP CTL in the H - 2b mice but only partial removal in H - 2d mice made to express thymic NP .