One of the first demonstrated effects of AHR activation by environmental contaminants was thymic atrophy, an AHR-dependent process.20 AHR can also suppress antibody production and the memory response to viral infections, resulting in lower levels of virus-specific IgG.353 The suppression of antibody responses extends to both primary and secondary immune responses, and in human population studies, exposure to AHR-binding POPs has been associated with lower responses to vaccination.354 AHR-mediated effects on antibody responses likely reflect the modulation of events in multiple cell types. This evidence concerns the gene AHR and thymus atrophy.