Iodine, long understood as a cornerstone of thyroid health, is now revealed to be a quiet architect of immune competence across the entire body — its presence or absence shaping how well living systems recognize, resist, and recover from threat. A narrative review from the University of Guelph and ImmunoCeutica Inc., published in Nutrients, finds that both deficiency and excess disturb immune homeostasis in ways current dietary guidelines were never designed to detect. The margin between nourishment and harm is narrower than science has yet mapped, and the work of mapping it — across species,