IL7 and lymphopenia: Interleukin-7 (IL-7) is required for T cell development and survival (reviewed in Khaled and Durum [1]) as first appreciated from the severe lymphopenia observed in IL7−/− and IL-7R−/− mice [2], [3], then in comparable deficiencies in humans found to lack components of the IL-7 receptor [4] (reviewed in Puel and Leonard [5]).