It has been thought that surface-expressed membrane-bound IgM is essential for B cell maturation and, hitherto, this type of IgM was also regarded as a prerequisite for B cell isotype switching. This view was supported by studies on mice with a targeted disruption of the membrane exon of the immunoglobulin μ gen (μMT mice); these mice are unable to express surface IgM and show B cell lineage cell development that is arrested at the pro-B(preB1) stage. More recently, however, Macpherson et al. reported that production, and isotype switching to, IgA can occur in μMT mice in the absence of both membrane-bound IgM and T cells. They found that serum IgA concentrations in these mice ranged from low to normal and increased with age, becoming constant after 6 weeks of life. Secretory IgA, induced by commensal intestinal bacteria or deliberate oral immunization, could also be readily identified in intestinal washes, which clearly suggests that another pathway for immunoglobulin isotype switching exists.
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