Total B cell percentages and proportions of the different B cell subsets [naive B cells (CD27 − IgD + IgM +), natural effector cells (CD27 + IgD + IgM +), total memory B cells (CD27 + IgD −), activated memory B cells (CD27 + IgD − IgM +), and class-switched memory B cells (CD27 + IgD − IgM −)] in healthy controls, smokers without airway obstruction, and COPD patients (GOLD stages I–IV) are shown in Fig. 2. Here, CD40LG is linked to chronic obstructive pulmonary disease.