IFNAR1 and infection: To corroborate whether MCs were able to degranulate in response to SFTSV in vivo, we examined serum levels of the MC-specific proteases tryptase and chymase in both WT mice (nonlethal SFTSV-infection model) and anti-IFNAR1 IgG-treated mice (lethal SFTSV-infection model) (Fig. 2A).