Preliminary studies on involvement of alpha-1-glycoprotein in the reaction of IgG-3 with leukocytes.

Alpha-1-glycoprotein (AGP) immobilized on Sepharose and human leukocytes, bound J125 IgG-3 and their fragments F(ab')2 at pH 7.0 and ionic strength 0.15. Increase in concentration of free AGP resulted in a release of the whole IgG-3 from AGP-Sepharose and a half of the bound IgG-3 from leukocyte surface. On the other hand, F(ab')2 fragments were released in both cases with 1% AGP. The binding of I125 F(ab')2 to leukocytes was suppressed by the whole non labelled IgG-3 molecules and their fragments F(ab')2. The remaining nonlabelled IgG and their fragments F(ab')2, despite their substantial excess, had no influence upon I125 F(ab')2 binding to leukocytes. AGP:I125 IgG-3 complex of the molar ratio 6:1 was shown to bind neither to leukocytes nor to AGP-Sepharose. A suggestion was put forward that AGP, which is anchored in leukocyte membrane, can participate in the binding of IgG-3 subclass. Instead, free AGP enables their release from cell surface.