The following points highlight the top four types of antigen-antibody interaction. The types are: 1. Antibody Affinity 2. Antibody Avidity 3. Cross-Reactivity in Antigen-Antibody Reaction 4. Precipitation Reaction.
Type # 1. Antibody Affinity:
The strength of the total non-covalent interactions between a single antigen-binding site (idiotope) on an antibody and a single epitope is the affinity of the antibody for that epitope. Antibodies with high affinity bind antigen more tightly and do not immediately dissociate from antigen where as antibody with low affinity binds weakly and can be easily dissociable.
The association between a binding site on an antibody (Ab) with a monovalent antigen (Ag) can be represented as:
The association constant ‘k’ is a measure of affinity which can be distinctly calculated from the ratio of the concentration of bound Ag-Ab complex to the concentration of unbound antigen and antibody like
The value of ‘k’ varies for different Ag-Ab complexes and depends upon both k1. K1 is expressed in litre/mole/second (l/mol/s) and k1 is expressed in 1 /second. The role of k1 in determining the association constant ‘k’ for several Ag-Ab interaction can be represented by Table 5.1.
A good example of showing affinity can be represented with small hapten’s, e.g., dinitrophenyl (DNP) conjugates and antibody binds to each other and each DNP group binds into antibody combining-site. Therefore, the small haptens are monovalent. The association constant k can be determined by equilibrium dialysis [Box 5.1 and Fig. 5.3].
Type # 2. Antibody Avidity:
When complex antigens having multiple, repeating antigenic determinants are mixed with multiple binding site antibodies, the interaction between antibody with antigen at one site will increase the probability of reaction at a second site. The affinity at one binding site does not always reflect the true strength of the antibody- antigen interaction.
The strength of such multiple interactions between a multivalent antibody and antigen is called the avidity. The avidity of an antibody is a better measure of its binding capacity within biological system. High avidity can compensate for low affinity. As for example— secreted pentameric IgM often has a lower affinity than IgG, but IgM has the high avidity.
Type # 3. Cross-Reactivity in Antigen-Antibody Reaction:
Antigen-antibody reaction is very specific but sometimes few antigens show cross reactivity with some unrelated antibodies. Cross reactivity occurs when two different antigens share an identical epitope and when a specific antibody for a specific antigen can also bind with other antigen with more or less similar structure of their epitope. Cross-reactivity can be well explained by the glycoproteins present on red blood corpuscles of ABO blood-group antigens, (Table 5.2) which are polysaccharide derivatives with different terminal sugar molecules.
A number of viruses and bacteria sometimes possess similar antigenic determinants to host-cell components. Antibody is raised by them by cross- reaction with host-cell antigens. As a result of this cross-reactivity sometimes auto-immune diseases start to damage tissue (Fig. 5.4).
Type # 4. Precipitation Reaction:
The interaction between an antibody and a soluble antigen in aqueous solution forms a lattice which ultimately develops a visible precipitate. Antibodies which can aggregate soluble antigens are designated as precipitins. It requires a time to develop a precipitate. The precipitate, attaining insolubility due to formation of ionic bond with antigen-antibody molecules, helps to lose its charge.
Ag-Ab lattice formation depends on the valency of both the antigen and the antibody, in this respect:
i. The antibody must be bivalent; a precipitate will not form with monovalent F(ab) fragments.
ii. The antigen must either be bivalent or polyvalent; it must have at-least two copies of the same epitope or have different epitopes that react with different antibodies present in polyclonal antisera (Fig. 5.5 and 5.6).