CD4 and COVID-19: Imeneo et al. used the QuantiFERON-TB Gold Plus (QFT-Plus) test (Qiagen, Germany), an interferon-γ release assay (IGRA), to detect the secretion of IFN-γ produced by lymphocytes specifically sensitized by Mycobacterium tuberculosis antigens and aspecifically by phytohemagglutinin (PHA) ex vivo: they found that in patients with severe COVID-19, the profound decrease in the peripheral blood T cell subsets (CD3+, CD4+, and CD8+) was related to an increase in QFT-Plus indeterminate rates, thus, reflecting a reduced IFN-γ production [12].