The majority of adult B lymphocytes in the mouse bear two immunoglobulin isotypes, IgM and IgD (mu(+)delta(+) cells) (1). A small population of IgM-bearing cells lacks, or expresses very low levels of IgD (mu- predominant [mup] cells) (1). These cells are believed to constitute a less mature subset of B cells analogous to neonatal B cells (2). Based on the time during ontogeny when responses to...
