A subpopulation of small pre‐B cells in rabbit bone marrow expresses x light chains and exhibits allelic exclusion of b locus allotypes

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Abstract
The expression of immunoglobulin heavy and light chains by pre‐B cells and B lymphocytes was examined by two‐color immunofluorescence in heterozygous b5b9 rabbits. Allelic exclusion of b5 and b9 x light chain allotypes was observed for both surface immunoglobulin‐negative pre‐B cells and surface immunoglobulin‐positive B lymphocytes. In newborn bone marrow, pre‐B cells and immature B lymphocytes expressing b9 were as numerous as those expressing b5. In contrast, circulating B cells and bone marrow plasma cells expressing the b5 marker outnumber b9+ cells by 2 to 1 in adult b5b9 animals. Whereas most B lymphocytes expressed x light chain b allotypes, approximately 80% of the μ heavy chain‐positive pre‐B cells did not. The pre‐B cells that expressed detectable light chains were relatively small lymphocytes. A model is presented which includes a “transitional” pre‐B cell that expresses both p chains and x chains.