“What’s the trick to the microbiome’s work with the immune system? Vitamin A — the bacteria moderate active vitamin A levels in the intestine, protecting the microbiome from an overactive immune response.”
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Paper: https://www.cell.com/immunity/fulltext/S1074-7613(18)30526-0
Commensals Suppress Intestinal Epithelial Cell Retinoic Acid Synthesis to Regulate Interleukin-22 Activity and Prevent Microbial DysbiosisGrizotte-Lake, Mayara et al.Immunity, Volume 49, Issue 6, 1103 – 1115.e6