Immunoglobulins M and D as antigen-binding receptors on the same cell, with shared specificity.

Antigen-binding cells specific for Type 3 pneumococcal polysaccharide were detected among normal human peripheral blood lymphocytes, tonsillar lymphocytes, and umbilical cord blood lymphocytes. By using mu and delta-specific antiserum it was shown that some specific antigen-binding cells simultaneously expressed both IgM and IgD on their surface. Both classes on individual cells were able to bind the same antigen, suggesting that the variable regions of each immunoglobulin class, when present on the same cell membrane, are coded by the same V gene.