The three constituents—latex-anti-aHCG, latex-anti-PHCG, and HCG molecules—were mixed simultaneously. The parameter of interest was the HCG concentration, which was set to 16.6, 33, 166, and 833 IU/g latex. Figure 10 represents the average masses S(t) and N(t) to increase as a function of aggregation time for aggregation experiments carried out in the presence of 33 IU/g antibody-coated latex particles. Two domains I and II of variation of S(t) and N(t) are observed, and the dashed line indicates the transition toward a slowing down of the aggregation rate after an initial period of 200 min. The global aggregation rate increases strongly with the HCG concentration because the time corresponding to the transition from domain I to domain II decreases from

FIG. 9 Representation of the reduced mass n/S(t) of aggregates present at the greatest concentration as a function of the concentration of IgM molecules expressed by IU/g IgG-coated latex.

10000

1000

FIG. 9 Representation of the reduced mass n/S(t) of aggregates present at the greatest concentration as a function of the concentration of IgM molecules expressed by IU/g IgG-coated latex.

2000 to 100 min when the HCG concentration increases from 16.6 to 833 IU/g latex.

The determination of the initial slope t of the aggregate mass distribution [Eq. (3)] provides direct information on the intrinsic reactivity of the colliding particles and aggregates. Results of Fig. 11 shows a strong decrease in t with the square root of the HCG concentration, and the value of t may serve to determine the HCG concentration without doing additional reference tests, as noted for the IgG-IgM system.

The decrease of the slope of t as a function of [HCG]05 leads to estimate m = 0.5 in Eq. (8) and 9 << 1 since the plateau of the electrophoretic mobility of the systems [latex-antiaHCG + HCG] and [latex-antipHCG + HCG] is only obtained for HCG concentrations greater than 5000 lU/g latex.

B. Heterocoagulation Involving One Latex Bearing the Complex [Antibody-Antigen] and the Second Latex Bearing Only the Second Antibody (33 IU/g Latex)

The complexes [antibody specific for the aHCG determinant + HCG (complex A) and [antibody specific for the PHCG determinant + HCG (complex B)] were

prepared by mixing the two sensitized latexes separately with HCG protein at the concentration of 33 IU/g latex and leaving the suspensions at rest for 24 h. Two types of aggregation experiments were carried out concomitantly. Complex A was added to the latex sensitized with the antibody specific for the PHCG determinant; likewise, complex B was added to the latex sensitized with the antibody specific for the aHCG determinant. Figure 12 represents the corresponding variations of S(t) (curve a) and N(t) (curve b) as a function of time (log-log scale). In both cases the initial fast regime abruptly changed to a slow regime. The complete similarity of the aggregation rates led us to conclude that the intrinsic reactivities of the latex bearing the antibody-HCG complex and the latex bearing only the antibody are similar for the two determinants. However, heterocoagulation in systems that contain the complex A (or B) and the other sensitized latex but no free HCG molecules produces aggregates of lower masses than does the experiment carried out with the system containing free

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