IL15 and lymphopenia: It is well-known that irradiation removes the sinks for homeostasis cytokines such as IL-7 and IL-15 [23] leading to IL-7 and IL-15 increase in lymphopenia [24, 25], and induces radiation-sensitive hematopoietic cell apoptosis, leading to releasing its IL-15Rα and subsequent formation of IL-15/IL-15Rα complexes [26, 27].