Heterogeneity of memory B cells in human blood

In human blood B cells exist in a variety of forms ranging from antigen-inexperienced naive cells to long-term memory cells, and antibody-producing plasma cells. Historically, the divisions between these types of cells have been defined by a few characteristic surface markers such as IgD, CD27, and CD21, but recent advances in single-cell techniques such as high-parameter cytometry and single-cell sequencing have revealed a greater diversity of populations. An expanding palette of markers has helped to better define B cell subsets but can be a source of confusion when overlapping cell types are identified using different sets of markers. In this review, we will discuss the conflicting and overlapping phenotypic and functional identities of human circulating B cells with a particular emphasis on memory B cells. We also propose a framework in which five B cell markers (IgD, CD27, CD21, CD11c, and a glycosylated isoform of CD45RB) can discriminate the majority of B cell populations in human blood.