AHR and immunodeficiency disease: The microbial metabolites of phenylalanine and tyrosine, which engage the aryl hydrocarbon receptor (AhR) and promote interleukin (IL)-22 secretion, and contribute to local immunization, were reported to be reduced in patients who developed ACLF, which manifested a relative immunodeficiency (Gao et al., 2018; Bajaj et al., 2020).