To explore the potential role of plasma cytokines in cTfh cell frequencies, the plasma levels of eight cytokines (IL-4, IL-10, IL-12, TNF-α, IL-17, IFN-γ, IL-2, and IL-6) in SLE patients and HCs were measured by a human CBA Kit using flow cytometry (Figure 4A), and plasma IL-21 levels were tested by ELISA in SLE patients and HCs. Here, IL21 is linked to systemic lupus erythematosus.