Notably, CXCR3+ CXR5+ memory Tfh cells are generated under some conditions, e.g. the seasonal influenza vaccine elicits ICOS+ PD‐1+ CXCR3+ CXCR5+ Tfh cells, and the circulating frequency of this population in the periphery correlates with the quantity and avidity of the influenza‐specific antibody response.158, 164. Here, ICOS is linked to influenza.