Studying correlative changes of Th1/Th2 (Th, Helper T cells) related factor Interferon-γ (IFN-γ) and Interleukin-4 (IL-4) in the progression of radiation pneumonia (RP) rats and the efficacy of Shashen-Maidong decoction on these indexes to explore the immune mechanism of the decoction on the prevention and treatment of RP. This evidence concerns the gene IL4 and radiation pneumonitis.