CD45+ cells in AF contained very low frequencies of T cells, B cells, and monocytes.Fetal CD103+ ILC3s in AF are functional and produce high levels of IL-17 and TNF. A similar subset was identified in second trimester fetal gut and lung, suggesting that CD103+ ILC3s develop in fetal tissues and subsequently egress to the AF. Here, IL17A is linked to atrial fibrillation.