CXCL13 and COVID-19: Additionally, increased serum levels of CXCL13 were observed in patients with severe COVID-19 compared to those with moderate disease and healthy controls, and the increased expression of CXCL13 was associated with increased antibody-secreting cell (ASC) production and activated circulating T follicular helper cells (cTfh), suggesting that CXCL13 plays an essential role in increasing the GC activity of patients with COVID-19 [61].