AHR and COVID-19: Interestingly, macrophages, dendritic cells, and neutrophils also showed IDO2 staining, but lymphocytes and mast cells did not [84]; moreover, since immunohistochemistry staining showed colocalization of IDO2 and several Trp metabolites, as well as AhR, Guo et al. hypothesize that systemic and early IDO2 expression and activity via AhR would result in fatal cellular stress in severe COVID-19 [84].