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Demonstration of mouse serum IgD.

Although the mouse would provide an excellent experimental model for the study of serum IgD function, previous investigations have failed to detect serum IgD in this species. We have developed an immunofluorescence inhibition assay that has allowed us to identify and quantitate mouse serum IgD. Mouse sera or serum fractions were assayed for their abilities to inhibit staining of mouse spleen cells with an allotype-specific fluorescein isothiocyanate-labeled rabbit antibody against mouse cell-surface IgD (FITC-RaMδ a ), as analyzed with a fluorescence-activated cell sorter. FITC-RaMδ a stains surface IgD + spleen cells of mouse strains that have surface(s) sIgD of the a allotype, such as BALB/c, but not strains that have surface IgD of the b allotype, such as C57BL/6 or C.B20, which have the C57BL/6 C H genome on a BALB/c background. The staining of BALB/c lymphocytes by FITC-RaMδ a was inhibited by BALB/c serum, but not by C57BL/6 or C.B20 serum. The staining was also not inhibited by BALB/c sera that had been adsorbed with rabbit anti-mouse Ig or a mouse hybridoma protein anti-mouse δ, or by BALB/c IgG, IgM, or IgA plasmacytoma proteins. These data strongly suggest that mice have IgD in their serum. Mouse serum IgD is probably secreted rather than shed from B cells, since amounts of serum IgD and splenic IgD in the same mice did not correlate with each other and serum IgD and shed sIgD had different density profiles.