Secretion of either of a pair of immunoglobulins, IgM or IgX, in somatic hybrid cells derived by fusion of a B-cell lymphoma cell line carrying both immunoglobulin isotypes.

The 1.29 cell line is a nonsecreting B-cell leukemia which bears two different immunoglobulin isotypes on its surface, IgM and IgX. The 1.29 cells were hybridized with nonsecreting myeloma cells giving rise to dozens of immunoglobulin secreting hybridomas. These fall into three groups differing in the class of immunoglobulin they secrete. Cells of the first group secrete pentameric IgM (mu, kappa), those of the second group secrete an unknown immunoglobulin, IgX, which may constitute an allotype of IgA, and those of the third group produce light chains only. The two complete immunoglobulins, IgM and IgX, have the same idiotype, as revealed by serological cross-reactivity of an exhaustively absorbed rabbit anti-idiotype serum. The molecular sizes of the heavy chains of the secreted IgM and IgX are slightly smaller than the mu and delta chains, respectively, which are derived from the surface of normal B cells as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.