A case with primary gastric lymphoma producing IgM and IgG immunoglobulins.

We describe a primary gastric lymphoma with IgM kappa and IgG kappa types of immunoglobulin in the neoplastic cells. The neoplastic cells containing IgM were morphologically different from those containing IgG. The IgG-bearing cells had globular cytoplasmic inclusions, but IgM-bearing cells did not. The neoplastic cells had two Ig heavy-chain gene rearrangements, with a germline band and a single kappa light-chain gene rearrangement. Immunoglobulin expression with light-chain restriction of the neoplastic cells and single light-chain gene rearrangement indicated that the morphologically different neoplastic cells were derived from a common progenitor. The expression of IgM and IgG and two rearrangement bands of immunoglobulin heavy-chain gene suggested that class switching might have occurred in the lymphoma cells.