IL21 and COVID-19: IL-21 secreted by TFH cells is critical in supporting B cell differentiation and antibody production.25,54 Therefore, to further explore the functional difference in spike-specific CXCR3+ and CXCR3− TFH cells, PBMCs from 12 COVID-19 convalescents, 13 vaccinees, and 11 healthy controls (Supplementary Table 4) were stimulated with spike peptide pools.