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Isolation of stable aggregates of IgG by zonal ultracentrifugation in sucrose gradients containing albumin.

Soluble heat aggregates of [125I]IgG (A-IgG) were prepared and separated by gel filtration or by zonal ultracentrifugation, and fractions containing different size aggregates were then analyzed by reultracentrifugation. A-IgG formed single narrow peaks with constant S rates when isolated, stored and analyzed in solution containing 0.5% serum albumin. Thus, stable homogeneous aggregates of IgG of known size can be simply prepared and should prove useful both as a model for immune complexes of specified sizes and as a standard for immune complex assays.