Allergic contact dermatitis is a delayed hypersensitivity reaction, which results from skin exposure to low molecular weight chemicals such as haptens. To clarify the pathogenic mechanism, electrospray ionization mass spectrometry （ESI-MS） and hydrogen/deuterium （H/D） exchange, as well as UV spectroscopy, were applied to determine the interaction between the model protein cytochrome c （cyt c） and the hapten 2,4- dinitro-fiuorobenzene （DNFB）. The ESI-MS results demonstrate that the conformation of cyt c can change from native folded state into partially unfolded state with the increase of DNFB. The equilibrium state H/D exchange followed by ESI-MS further confirms the above results. UV spectroscopy indicates that the strong- field coordination between iron of heme （prosthetic group） and His18 or Met80 of cyt c is not obviously affected by the hapten.
