We have shown that 8 out of 10 HLA DRB1*1501 healthy adults have detectable StkP446–60-tetramer binding CD4+ T-cells with a range in frequency (0.01–0.07%) similar to frequencies observed in other models of exposure to viral and intra-cellular bacterial infections as well as following desensitisation therapy in allergic disease [16], [17], [18], [19]. Here, CD4 is linked to bacterial infectious disease.