Suboptimal non-inflammatory concentrations of haptens may elicit a contact sensitivity reaction when used as a mix

Contact dermatitis is a cutaneous inflammatory reaction mediated by hapten-specific T cells. We used a murine model to investigate the contact sensitivity reaction elicited by different concentrations (optimal and suboptimal) of the haptens DNFB and oxazolone applied singly or in combination. The simultaneous application of DNFB and oxazolone at optimal concentrations (0.2% and 0.4% respectively) did not significantly increase the ear swelling response induced by each of the allergens when applied singly. No contact sensitivity response was observed when the haptens were tested individually at subthreshold concentrations (0.05% and 0.1% respectively). However, mixing the 2 molecules at the same concentrations gave rise to a clinical contact sensitivity reaction. The simultaneous application of the haptens at a 2 x higher, but still suboptimal concentrations (0.1% and 0.2% respectively), elicited an inflammatory response that was significantly greater than the responses elicited by either of the haptens when applied separately. These results suggest that a "false-positive" reaction to a mix may reveal a genuine sensitization to the constituents.