IFNG and COVID-19: Zheng et al. [29] noted that in critical forms of COVID-19, the number of polyfunctional T cells (producing more than one cytokine) was reduced in parallel with the production of IFN-γ, TNF-α, and granzyme B. Another study [69] demonstrated an increase in the granzyme B and perforin levels in CD8 T cells of critically ill patients.