IFNG and COVID-19: Through stimulation of these TFH cells by agonism of the ICOS receptor that would produce normal synthesis of IFN-γ/IL-21, it was observed in samples from hospitalized COVID-19 subjects that TFH cells (CD3+ CD4+ CXCR5+ ICOS+) within ambulatory subjects encompassed, on average, 15% TFH of the peripheral blood CD4+ compartment relative to the normal 8% TFH population [366].