AHR and infection: For example, in human population studies, there is limited or no association between exposure to AHR-binding pollutants and the steady-state frequency of circulating leukocytes34,256; however, the severity of infection is inversely related to exposure.257,258 Many in vivo studies have shown no discernable impact of AHR ligand treatment on the immune system in the absence of stimulating immune cells (e.g., by adding an antigen or mitogen).