CD27 and viral infectious disease: Since the more broadly-defined Double Negative B population was initially described as an antigen-experienced memory B cell population lacking expression of CD27 with nearly half of the population class-switched to IgG (5), and the majority of DN1 cells from heathy controls express IgG (9), we asked whether viral infection also modified isotype class dominance in DN1, DN2 and DN3 subsets.