The principal findings in the skin during the first 24 hr were invasion of polymorphs and destruction of pilosebaceous units, in both intact and thymectomized mice. Subsequently, the dermal cellular infiltrate increased and there was acanthosis of the epidermis. No lymphocytes were seen in the dermis or penetrating the epidermal basal cell layer in thymectomized mice.

During the first 24 hr in the draining node, polymorphs and macrophages bearing a pigment with staining properties similar to melanin were seen in the marginal and medullary sinuses, in intact and thymectomized mice. Major differences, however, were revealed during 2–4 days when massive proliferation of large pyroninophilic blast cells occurred in the thymus-dependent area of the nodes from intact mice only.

On testing, there was a prompt, measurable increase in ear thickness only in intact mice. This increase reached a peak at 24 hr — typical of a delayed type reaction. At testing, the ears from intact mice showed epidermal vesiculation and a considerable dermal cellular infiltrate with a substantial number of lymphocytes. This was in contrast with the completely quiescent appearance of the ear skin of thymectomized mice.

Finally, we have discussed the use of the mouse as an experimental tool for studying contact sensitivity and have analyzed the role of the thymus-derived lymphocyte and the site where it becomes sensitized, in the light of current theory on the origin of cells and site where sensitization takes place in cell-mediated reactions.