Also, a shift of the B cell compartment toward “atypical” memory B cells (lacking CD27 and low CD21 expression, unlike classical B cells that are CD27+ CD21+), described in other viral infections such as HIV, HBV, or HCV, have been described in COVID-19 patients (64), but the exact role of these atypical B cells needs to be clarified. Here, CD27 is linked to viral infectious disease.