IL4 and cystic fibrosis: In CF patients, high systemic and pulmonary expression of the T helper cell 2 (Th2) cytokine interleukin 4 (IL-4) correlates with chronic P. aeruginosa infection and poor pulmonary function, while high expression of the Th1 cytokine interferon-γ (IFN-γ) correlates with improved pulmonary function [5–7].