B-1 cell

B-1 cells are B cells that express CD5, which can bind to another B cell surface protein, CD72. CD5-CD72 is thought to mediate B cell-B cell interaction. B-1 cells express IgM in greater quantities than IgG and its receptors show polyspecificity, meaning that they have low affinities for many different antigens, but have a preference for other immunoglobulins, self antigens and common bacterial polysaccharides. B-1 cells are present in low numbers in the lymph nodes and spleen and are instead found predominantly in the peritoneal and pleural cavities. B-1 cells generate diversity mainly via recombinatorial recombination (there is a preferential recombination between D-proximal VH gene segments). B-1 cells are first produced in the fetus, unlike conventional B-2 cells that are produced after birth and replaced in the bone marrow.

Isolation of peritoneal B1 cells
In research laboratories, B1 B cells can be easily isolated from a mouse by injecting cell media or PBS into the peritoneal cavity of the mouse and then draining it off via a technique mirroring diagnostic peritoneal lavage. Cells can be identified and placed into 2 categories "B1a" or B1b" using multi-colour Flow cytometry looking for surface expression of CD19, B220, and CD5. B1a expresses high CD5 level, while B1b expresses low CD5 to almost-absent levels; both are CD19+ and B220+.