Differences between activated and resting cTfh cells have largely been characterized using cell surface markers, with the activation markers ICOS, CD25, CD27, CD28, CD38, CTLA4, PD-1, Helios, and Ki67 shown to have higher surface expression on activated compared to resting cTfh cells following influenza vaccination (8, 19). This evidence concerns the gene MKI67 and influenza.