IFNG and COVID-19: In the context of an early immune response against SARS-CoV-2, a major source of IFN-γ is likely to be mucosa-associated invariant T (MAIT) cells as these cells are activated and substantially depleted from blood in patients with severe COVID-19 (66, 67) and, furthermore, a fatal outcome of COVID-19 was associated with higher production of IFN-γ, compared with other cytokines, by MAIT cells (67).