CD27 and infection: We next assessed the expression of CD150 on further defined circulating B-cell subsets, transitional, non-class-switched IgM-positive (IgM+) memory (IgM-only and natural effector B-cell) subsets, class-switched CD27− IgM-negative (IgM−) and CD27+ IgM− memory (IgG+ and IgA+) subsets, and plasma cells, and also their MV infection levels.