Cytoplasmic immunofluorescence of bone marrow plasma cells producing immunoglobulins of the four IgG subclasses.

Immunofluorescence studies were performed on bone marrow samples from 21 individuals with normal or slightly elevated serum IgG levels. On the average, 40.7 percent of the IgG-positive plasma cells contained molecules of the subclass IgG; 30.6 percent had intracellular IgG2; 21.7 percent, IgG3, and 7.0 percent, IgG4. These figures were compared to the relative serum IgG subclass concentrations. The principal finding was the discrepancy between the high relative number of IgG-3 producing bone marrow plasma cells and the low relative IgG3-producing bone marrow plasma cells and the low relative IgG3 serum concentration (5.5 percent of the total IgG). It was explained by the short biologic half-life of molecules of this subclass.