There is a subset of B cells, called B-1 B cells, that arise before B-2 B cells, the major group of B cells in humans and mice. In humans and mice, B-1 B cells compose about 5% of the total B-cell population. They appear during fetal life, express surface IgM but little or no IgD, and are marked by the display of CD5. However, CD5 is not an indispensable component of the B-1 lineage, it does not appear on the B-1 cells of rats, and mice that lack a functional CD5 gene still produce B-1 cells. In animals whose B-2 B cells are the major B-cell population, B-1 cells are minor populations in such secondary tissues as lymph nodes and spleen. Despite their scarcity in many lymphoid sites, they are the major B-cell type found in the peritoneum.
Although there is not a great deal of definitive information on the function of B-1 cells, several features set them apart from the B-2 B cells of humans and mice. During fetal life, B-1 cells arise from stem cells in bone marrow. However, in postnatal life this population renews itself by the prolifer ation of some B-1 cells in sites outside the bone marrow to form additional naive B-1 cells. The B-1 population responds poorly to protein antigens but much better to carbohydrate ones. Most of its members are IgM-bearing cells, and this population undergoes much less somatic hypermutation and class switching than the B-2 set of B cells does. Consequently, the antibodies produced by a high proportion of B-1 cells are of rather low affinity.
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