2). The frequency of CD27-IgD- memory B cells increased during the first years of age and did not show further age-related changes. The frequency of CD21lowCD38low B cells increased slightly with age (Fig. 2). Plasmablasts were rarely detected click here in the peripheral blood (Fig. 2). The absolute count of distinct B cell subsets is dependent upon the relative frequency of each B cell subset as well as upon the developmental changes of the total B cell count. The number of total B cells decreased with increasing age. Within the B cell pool, absolute

counts of naive and transitional B cells decreased with increasing age, with the strongest decline in the first 5 years of age (Fig. 3). Whereas the absolute number of switched memory B cells increased slightly with age, the number of non-switched and CD27- memory B cells decreased during the first 5 years of age and was stable thereafter. The latter was also the case for the absolute numbers of CD21lowCD38low B cells and plasmablasts (Fig. 3). Age-dependent changes of B cell subpopulations and total B cell numbers were most obvious within the first 5 years of life. Therefore, the cohort of 220

individuals was divided into seven age groups. The frequency and the total number of distinct Inhibitor Library concentration B cells are shown as median values as well as the interquartile ranges (25th and 75th percentiles) in Tables 1 and 2. Immunofluorescent staining approaches using separated PBMCs and whole blood have been directly compared for all B cell subsets in 21 individuals. The counts of each B cell population showed Mannose-binding protein-associated serine protease a close correlation between both approaches (Fig. 4). Additionally, we compared the frequency of CD19+ B cells using two gating strategies for the lymphocyte gate: forward-/side-scatter and CD45/side-scatter. The frequency of B cells showed a close correlation between both gating strategies in these patients. This was noted for the whole blood staining

approach (r = 0·98, P < 0·001) and the PBMC approach (r = 0·99; P < 0·001). Several new B cell populations have been characterized in the last years which have been suggested to develop in an age-dependent manner [5,6,8–13,17,21,22]. Additionally, distinct patterns of disturbed B cell homeostasis or impaired B cell development have been characterized in several immunological diseases [14,18,23]. However, age-dependent reference values for a distinct B cell population are rarely reported [19,20]. Therefore, we have characterized developmental changes in distinct peripheral B cell populations from infancy to adulthood and generated age-dependent reference values. Most attempts to characterize peripheral B cell populations have concentrated upon the delineation of distinct developmental stages. The earliest B cell stage which can be detected in the peripheral circulation has been termed ‘transitional B cell’ or ‘recent bone marrow emigrant’[11–13,22]. Several flow cytometric approaches have been suggested to characterize this B cell population.