Table 1. Summary of Murine B-Cell Subsets and Their Surface Markers, Localization, and Frequencies in the Spleen.

Murine B Cells	Cell Surface Markers	Localization	Frequency (of Splenic B Cells)
B1
B1a	CD19⁺ B220^low IgM^high CD5⁺ CD43⁺ CD23⁻	Spleen, peritoneum	≈2%
B1b	CD19⁺ B220^low IgM^high CD5⁻ CD43⁺ CD23⁻	Spleen, peritoneum	<1%
IRA
Innate response activator	IgM^high CD43⁺ CD23^low CD93⁺	Spleen	<0.5%^*
B2
Marginal zone	CD19⁺ B220⁺ IgM^high CD21^high CD43⁻ CD23⁻	Spleen	≈10%
Follicular	CD19⁺ B220⁺ IgM^low CD21⁺ CD43⁻ CD23⁺	Spleen, circulation	≈80%
Bregs
B regulatory	CD19⁺ B220⁺ IgM^high CD1d^high CD43⁻	Spleen	≈1%

Bregs indicates regulatory B cells; and IRA, innate response activator.

The frequency of IRA B cells increases to ≈4% in response to lipopolysaccharide injections.