Fig. 4.

High background staining correlates with a low detection rate in phase 3. The % of IFNγ-producing cells in the negative control sample (i.e., background) versus the detection of a response in the corresponding stimulated sample is depicted for all donor–antigen combinations (and all participants) in which a positive response should have been detected (left; n = 50 stainings) or in the case that low-frequency responses (as observed against FLU; n = 30 stainings) should have been detected (right). The background was significantly lower in those donor–antigen combinations where a response was detected