IL4 and Recurrent lower respiratory tract infections: Studies have shown that compared with nonsmokers, smokers with lower respiratory tract infection and health examination can increase Il-4 and IL-5 in induced sputum and decrease INF-γ, and increased smoking index can inhibit the expression of INF-γ, imbalance of TH1/TH2 system, and disorder of internal immune defense mechanism, affecting the normal immune response after infection [25].