to them twice a day and this for four days on the fifth day the product is applied a fifth time to said explants that are then subjected to a 7 j cm 2 u v a radiation at the same time as the explants of the second series the three series of explants are then frozen by immersion in isopentane that is cooled to u2212 55 u00b0 c in liquid nitrogen there are then made approximately ten frontal slices of 6 u03bc inside a cryostat at a temperature of u2212 16 u00b0 c this having as its effect to stop the immunity activity of the langerhans cells there is then proceeded to the incubation of a primary antibody and of a secondary antibody in each explant which antibodies make it possible to reveal by means of the immunofluorescence technique the activity of the langerhans cells indeed the langerhans cells are active only when they play an antigenic part by blocking the antibodies it being possible to observe this antigenic action under a fluorescence microscope the primary antibody is chosen so that it recognizes the langerhans cells and the secondary antibody coupled with fluorescent molecules of tetramethyl rhodamine isothiocyanate tritc is provided to color these cells the cut up and incubated samples are.