A C. burnetii-specific interferon-γ (IFN-γ) recall assay—involving (i) ex-vivo peripheral blood mononuclear cells (PBMCs), (ii) C. burnetii in-vitro antigen stimulation (varying from overnight to 48 h) and (iii) subsequent IFN-γ detection in the supernatant—was successfully deployed in experimental Q fever research settings (18, 19). This evidence concerns the gene IFNG and Q fever.