IDO1 and COVID-19: A low expression of the IDO1 enzyme decreases the activity of the aryl hydrocarbon receptor (AhR), which is a cytosolic nuclear receptor and transcription factor that, when activated by ligands, such as kynurenine metabolites, translocates to the cell nucleus and regulates the expression of genes involved in the immune response, in the homeostasis of mucosal barriers, and in the detoxification of xenobiotic compounds, thus being crucial for maintaining mucosal barriers such as the blood–brain barrier, and explaining the neurological sequelae of COVID-19.