Cytokine response elicited by PPD (panel A and B) or Ni (panel C and D) by ELISpot analysis in patch test negative or positive patients, respectively. ELISpot results of detection of IFN-γ (panel A-C) or IL-10 (panel B-D) producing PBMC upon stimulation with Ni (20 μg/ml) or PPD (10 μM) were expressed as the mean values of S.I. ± SD. S.I. Index value is expressed by the ratio between the number of spot forming cells (IFN-γ and IL-10 producing PBMC) upon stimulation with the allergen and those present in the absence of stimuli (spontaneous cytokine production). Positive IFN-γ-responses (Index ≥ 3), in the presence of PPD or Ni, were detected only in subjects with positive patch test to PPD (panel A, P = 0.002) or to Ni (panel C, P = 0.001), respectively.