Calculation of the affinity constant KASS for solid phase antigen. A model system using monoclonal antibodies against the cat allergen Fel d I

Structural aspects of cross-reactivity and its relation to antibody affinity

In the context of IgE/allergen interactions, affinity is largely determined by the stability of the allergen-IgE complex: a low affinity is usually equated with a rapid dissociation of the complex. Regular solid-phase assays are not well suited for affinity estimates because of multivalency effects, "unstirred layer" effects and "invisible" antibodies. Elution of IgE bound to solid-phase coupled allergen might be a good measure of intrinsic affinity, provided that reassociation of antibodies is prevented by a high concentration of soluble allergen. Allergen-mediated IgE-dependent triggering of a mast cell is presumably a two-step process. During the first step, the allergen is bound to a cell-bound IgE antibody and dragged over the cell surface. The second step is the interaction between this cell-bound allergen and another IgE antibody. The hypothesis is that the affinity requirements for the first step are higher than for the second. The implication is that a mast cell can be triggered by a single high-affinity antibody in combination with one or more low-affinity antibodies.

Affinity of IgE antibody to antigen influences allergen-induced histamine release

BACKGROUND

Although affinity of an antibody for an antigen is recognized to be an important factor in determining its biological effects, little is known about the relevance of such affinity of IgE antibodies to the functional response.

OBJECTIVES

To investigate the effect of IgE antibody affinity to Der p 2 on Der p 2-induced histamine release from human basophils.

METHODS

The most probable value of the dissociation constant (Kd) of IgE antibody to Der p 2 was calculated and histamine release by Der p 2-challenged leucocytes was used to evaluate the biological efficacy of the IgE antibody.

RESULTS

The most probable Kd value of IgE antibody to Der p 2 ranged from 5.6 to 177.8 pM in 14 asthmatic patients sensitive to Der p 2. A significant correlation was observed in Der p 2-induced histamine release between the sensitivity and the Kd value for Der p 2-specific IgE antibody (rs = 0.797, P = 0.0040), suggesting that the higher the affinity, the lower the amount of allergen required for the release of a specific level of histamine.

CONCLUSION

Apart from the changes associated with the reactivity, the sensitivity of histamine release is closely related to the affinity of IgE antibody for its antigen.

Differences between specificities of IgE and IgG4 antibodies: studies using recombinant chain 1 and chain 2 of the major cat allergen Felis domesticus (Fel d) I

IgE- and IgG4 antibodies were compared for reactivity with recombinant chain 1 and chain 2 of the cat allergen Felis domesticus (Fel d) I. Recombinant chain 1 and chain 2 were coupled to sepharose and tested in IgE- and IgG4 radioallergosorbent test (RAST) experiments. Substantial IgE- and IgG4 binding was found. The fraction of Fel d I-specific antibody that bound to the recombinant chains was calculated. For chain 1, the mean value of this fraction was 0.30 for IgE and 0.23 for IgG4 (P = 0.05). For chain 2, the mean value of this fraction was 0.19 for IgE and 0.13 for IgG4 (P = 0.02). These results indicate that differences in fine specificity exist between IgE and IgG4 antibodies. Moreover, these findings support our results with chemically prepared peptides derived from these two chains and suggest that the B cells producing IgE antibodies are more likely to recognize a less 'native' form of Fel d I, compared with IgG4.