CCL2 and chronic obstructive pulmonary disease: Additionally, studies of COPD cohorts often analyze levels of circulating serum or plasma to compare with pulmonary function data and have found increased levels of mediators such as MCP‐1, IL‐6, and IL‐13 to be associated with increased disease severity and poorer lung function, further indicating that sampling region may play a role in varying results (Di Stefano et al., 2018; Lee et al., 2007).