We propose a comprehensive mechanism for the formation and growth of <100> interstitial loops in alpha-Fe. This mechanism reconciles long-standing experimental observations of these defects in irradiated ferritic materials with recent atomistic simulations of collision cascades and defect cluster properties in Fe, in which highly mobile 1 / 2<111> clusters are seen to be the dominant feature. Hence, this work provides one of the necessary links to unify simulation with experiments in alpha-Fe and ferritic alloys subject to high-energy particle irradiation.