The impaired differentiation of B cells was also confirmed by the reduction of ‘naive’ IgD+CD27− cells, transitional IgD+CD27−CD10+CD45RB− cells, and follicular CXCR5+ (IgD+CD27−CD10−CD73+) cells in the peripheral blood of patients with severe COVID-19, as compared to convalescent COVID-19 patients and healthy controls [54]. Here, CD27 is linked to COVID-19.