An immature B cell that is IgM-positive, CD45R-positive, CD43-low, CD25-negative, and CD127-negative. This cell type has also been described as being AA4-positive, IgM-positive, CD19-positive, CD43-low/negative, and HSA-positive. [ http://www.ncbi.nlm.nih.gov/pubmed/18432934 http://www.ncbi.nlm.nih.gov/pubmed/17582343 GOC:dsd GOC:ak GOC:tfm ]