Evidence of a two stage nature of precipitin reactions

The influence of epitope density on the estimation of the affinity of antibody for complex antigens

The influence of the epitope density of the antigen on antibody affinity values determined by fluid- and solid-phase immunoassays was assessed. The affinity of the interaction of a panel of monoclonal anti-DNP antibodies of different affinities (as determined by equilibrium dialysis) for DNP-protein conjugates of various hapten substitution ratios was used as the test system. The results obtained showed that the epitope density of the antigen markedly influences the observed affinity values obtained by both experimental approaches. However, the monoclonal antibodies were ranked in affinity terms by both assays in a similar order to that given by equilibrium dialysis. It is concluded that provided due care is exercised in choosing an appropriate epitope density for the test antigen, these methods can be used to provide rapid estimations of average antibody affinities.

A general theory for the precipitin reaction--based on Fc-mediated precipitation

Fc-mediated immune precipitation is studied theoretically by computer simulation of precipitin curves and of zonal profiles of soluble immune complexes. Comparison with experiments indicates that Fc-mediated immune precipitation is caused by Fc-Fc interactions with association constants ranging from 10(5) to 10(7) liters/mole. The Langmuir adsorption isotherm is used for evaluation of these interactions. A general theory for the precipitin reaction is formulated, assuming it to be a three-stage process: 1st stage Formation of immune complexes, 2nd stage Precipitation due to coalescence of antibody-rich immune complexes, and 3rd stage Fc-mediated immune precipitation.

Influence of antibody affinity on the performance of different antibody assays

The effect of antibody affinity on the performance of 5 commonly used assays was studied. The assays used were measurement of antigen binding capacity in a Farr type assay, haemagglutination, solid-phase radioimmunoassay (SP-RIA), solid-phase ELISA and precipitation. The first 4 assays were all more sensitive for high affinity antibodies. Precipitation was not related to affinity, suggesting that factors secondary to antigen-antibody binding may be more important in determining the level of precipitate formation. The effect of epitope density of the antigen was also investigated in the SP-RIA and ELISA. Affinity dependence was more marked when antigen of low epitope density was used and this dependence could be reduced in the ELISA by choosing a low OD endpoint. Thus, the most reliable way to estimate antibody content by the ELISA may be to determine a low OD endpoint titre against antigen of high epitope density. When epitope density per molecule cannot be increased, an alternative approach to the problem is to increase epitope density by covalent coupling of antigen to the solid-phase rather than by adsorption.

Binding of complement subcomponent C1q to mouse IgG1, IgG2a and IgG2b: a novel C1q binding assay

A C1q binding assay is presented which is suitable for use in comparison of the binding ability of different antibodies, and which allows the quantitative determination of their binding constants. The assay system uses IgG bound to a hapten-derivatized Affigel support. No non-specific binding is observed to a DNP-derivatized support, allowing the use of anti-DNP antibodies. With mouse anti-DNP hybridoma IgGs it was found that C1q binding followed the series IgG2a greater than IgG2b much greater than IgG1, in accordance with the complement fixing ability of these subclasses. Since it is relatively simple to couple any antigen to Affigel , this assay system should be generally applicable to any antibody-antigen system.

The significance of antinuclear antibodies in juvenile rheumatoid arthritis associated with chronic bilateral iridocyclitis

Serum samples from 8 children with juvenile rheumatoid arthritis (JRA) and chronic bilateral iridocyclitis were significantly distinguished from 5 children with JRA and no eye symptoms by the presence of large immune complexes (IC) greater than 22S, IgM antinuclear antibodies (ANA), IgG granulocyte-specific (GS-) ANA, C3 fixing ANA, and IgM anti-IgG. One serum with and two sera without IC greater than 22S, all from patients with iridocyclitis, were fractionated by rate zonal ultracentrifugation. Each fraction relevant for the study was separately concentrated and reexamined. In one of the sera without IC greater than 22S this technique exposed the presence of IgA GS-ANA not detectable in the corresponding whole serum. IgG ANA were precipitated in an area with higher molecular weight than the one for IgG indicating the presence of aggregated IgG ANA. Fractionation of the serum with IC greater than 22S demonstrated IgM GS-ANA not present in whole serum. The results support previous suggestions that ANA may be involved in the pathogenesis of chronic iridocyclitis and may explain why ANA (in particular C3 fixing ANA) negative patients with JRA rarely develop chronic iridocyclitis.

Biochemical aspects of immune complex formation and immune complex diseases

Formation of immune complexes is a normal part of the immune defence against soluble antigens. Immune complexes may nevertheless play a pathogenic role of their own. The present review discusses antigen antibody interactions with special regard to immune complex formation. A new classification of antigen antibody interactions is proposed, based on the antigenic valence. Oligovalent antigens and bivalent antibodies form genuine immune complexes. Experimental observations and theoretical considerations indicate that although a vast variety of complexes is possible genuine immune complex formation is a self-limiting process, so it is typical that the smallest possible complexes are formed in the largest amounts. Formation of immune complexes differs in mechanism from most other biological ligand binding reactions, and the extent to which immune complex formation follows its own laws is discussed. To explain the mechanism of precipitin reactions, a two-stage model is proposed. The outcome of antigen antibody interactions is further complicated because antibody preparations are typically heterogeneous, and the impact of antibody heterogeneity is also discussed.