The formation of insoluble immune complexes between ovalbumin and anti-ovalbumin IgG occurs in at least two distinct phases dependent on reactant concentration and ionic strength.

The mechanism regulating the formation of insoluble immune complexes (IIC) in serum in certain disease states is not well understood. Ovalbumin and rabbit anti-ovalbumin IgG was used to study the formation of IIC in vitro in a stirred reaction vessel; and the radii of IIC that formed was determined by light scattering techniques. Using an initial IgG concentration of 1 mg/ml at equivalence antigen:antibody ratio IIC formation was detected within 5 s, and the complexes increased in radii to approx. 100 nm after 20-30 s (phase 1). This was followed by a phase (phase 2) in which the complexes rapidly increased in radii to the point where Mie scattering was reached (approximately 200 nm). The time of onset of the second phase decreased with increasing initial IgG concentrations at a fixed antigen:antibody ratio; and was at a minimum at equivalence antigen:antibody ratio, but increased at both antigen and antibody excess ratios. Immune complexes formed using F(ab')2 fragment showed a similar pattern to those formed using IgG. A similar pattern was seen in the presence of the complement component C1q which potentiated IIC formation in phase 2, and human serum (1:10 dilution) which attenuated IIC formation in both phases. For complex formation using IgG and ovalbumin the presence of NaC1 at concentrations up to 0.6 M led to a progressive increase in the time of onset of phase 2; potencies of inhibition by other sodium halides followed the lyotropic series NaF < NaC1 < NaI. The results suggest that formation of IIC occurs in at least two distinct phases, and that the second phase leading to the generation of very large insoluble complexes is associated with a rapid polymerisation of the complexes by a mechanism that is not dependent on Fc:Fc interactions.