GZMB and COVID-19: A deficiency in peripheral blood MAIT cell frequencies has consistently been reported by others also (60-67), again associated with increased surface expression of activation markers CD38, CD69 and HLA-DR, and with increased production of IL-17, TNF and granzyme B. MAIT cells in COVID-19 patients also upregulated the exhaustion markers CTLA-4 and PD-1(65), as is observed with chronic exposure to viral antigens(68) or inflammation, and associated with hierarchical loss of effector function, followed by activation induced cell death(69).