One critical growth factor for the differentiation, proliferation and activation of AMs is GM-CSF (71–73) which is largely produced by type II alveolar epithelial cells during influenza (74, 75) and mice deficient in GM-CSF (Csf2−/−), or its receptor (Csf2rb−/−) have increased morbidity and mortality during influenza similar to animals that are devoid of AMs (76) (Figure 2). Here, CSF2RB is linked to influenza.