Which of these are found on a mature B cell?

• A. IgG and IgD
• B. IgM and IgD
• C. Alpha and Beta chains
• D. CD3

Answer: (B)

Mature B cells carry their antigen receptor as membrane-bound immunoglobulin, and the isotypes expressed on the surface are IgM and IgD. In naive mature B cells, IgM and IgD are coexpressed as the B cell receptor, produced by alternative splicing of the same heavy chain transcripts. IgG is the product of class switching that occurs after activation, so it is not present on naive mature B cells. The other options point to components not used by B cells: Alpha and Beta chains are parts of the T cell receptor, and CD3 is part of the TCR complex found on T cells, not B cells.