Abstract: Membrane anchoring peptides are attached to the C terminal end of the heavy chain of the various immunoglobulin isotypes (IgM, IgD, IgA, IgE, or IgG). The membrane anchoring peptides span the cell membrane lipid bilayer of B cells thereby affixing the associated immunoglobulin to the cell membrane surface. The extracellular segments of these peptides are unique for different isotypes. Epitopes unique to the B cells which produce each isotype are formed, in whole or in part, by these extracellular segments. These membrane-bound immunoglobulin isotype-specific ("migis") extracellular epitopes are not present on the secreted, soluble form of the immunoglobulins, which are not bound to the cell surface by the membrane anchoring peptides. The antibodies of the invention (and other related products) specifically bind to the extracellular migis epitopes of human .mu. chain, human .delta. chain, or human .gamma. chain.