One of the fundamental effects of irradiation is accelerated electron transfer in the respiratory chain of the mitochondria. As a consequence increased ATP production results. This activates a large number of downstream reactions. Another consequece of the enhanced electron transfer in the respiratory chain is the leakage of the superoxide anion which produces additional reactive oxygen species (ROS). The ROS activate the nuclear factor NF kappa B, whuch is involved in a number of important cell reactions For further details consult the papers by M. Hamblin and his coworkers on the internet (Google).