The understanding of helium effects in synergy with radiation damage is crucial for the development of structural steels for fusion applications. Recent investigations in ultra-pure iron, taken as a basic model, have shown a drastic impact of dual beam (helium and iron) exposure on the accumulation of radiation-induced dislocation loops in terms of strong bias towards a(0)/2 < 111 > loops, while a(0)< 100 > loops are mostly observed upon iron ion beam. In this work we perform a series of atomistic studies to rationalize possible mechanisms through which He could affect the evolution of microstructure and bias the population of a(0)/2 < 111 > loops. It is shown that He atoms are dragged by gliding a(0)/2 < 111 > loops. This strongly suppresses loop diffusivity and in turn it prohibits the mutual interaction of a(0)/2 < 111 > loops, being prerequisite of the formation of a(0)< 100 > loops, as well as it reduces the disappearance of a(0)/2 < 111 > loops at sinks. A scenario for the microstructural evolution in the single-and dual-beam conditions is discussed. (C) 2014 Elsevier B. V. All rights reserved.