CCL2 and infection: During infection, alveolar epithelial cells, endothelial cells, and resident macrophages secrete cytokines, notably, CCL2, MIP-1α, KC/IL-8, G-CSF, GM-CSF and IL-1β that recruit and activate circulating neutrophils to migrate into the alveoli, damaging the alveolar-capillary barrier by releasing ROS and proteases that damage endothelial and epithelial cells, and allowing influx of fluid into the air spaces causing hypoxemia [43].