CR2 and infection: As reported in human patients, we divided B cells into resting (CD21+CD27+), activated memory B cells (CD21−CD27+), switched memory B cells (CD27+IgD-), unswitched memory B cells (CD27+IgD+), naive memory B cells (CD27-IgD+), and double-negative B cells (CD27-IgD-) and assessed the changes of these subsets during infection (20).