The interactions between antigens and antibodies are known as antigen–antibody reactions. The reactions are highly specific, andan antigen reacts only with antibodies produced by itself or with closely related antigens. Since these reactions are essentially spe-cific, they have been used in many diagnostic tests for the detec-tion of either the antigen or the antibody in vitro. The antigen and antibody reactions also form the basis of immunity against microbial diseases in vivo. In the host, it may cause tissue injury in hypersensitivity reactions and in autoimmune diseases.
General Features of Antigen-Antibody Reactions
Antigen and antibody bind through noncovalent bonds in a manner similar to that in which proteins bind to their cellular receptors, or enzymes bind to their substrates. But antigen– antibody reactions differ from the latter as there is no irreversible chemical alteration in either of the participants, i.e., antigen or the antibody. The antigen and antibody binding is reversible and can be prevented or dissociated by high ionic strength or extreme pH. Following are some of the general features of these interactions:
Electrostatic bonds, hydrogen bonding, van der Waals bonds, and hydrophobic interactions are the intermolecular forces involved in antigen–antibody reactions. All these types of intermolecular forces depend on the close proximity of the antigen and antibody molecules. For that reason, the “good fit” between an antigenic determinant and an antibody-combining site determines the stability of the antigen–antibody reaction. Multiple bonding between the antigen and the antibody ensures that the antigen will be bound tightly to the antibodies.
Affinity denotes the intensity of attraction between antigen and antibody.
· Low-affinity antibodies bind antigen weakly and tend to dis-sociate readily, whereas high-affinity antibodies bind antigen more tightly and remain bound longer.
· High-affinity binding is believed to result from a very close fit between the antigen-binding sites and the corresponding antigenic determinants that facilitates development of strong noncovalent interactions between antigen and antibody.
Avidity is a measure of the overall strength of binding of an antigen with many antigenic determinants and multiva-lent antibodies. Avidity is a better indicator of the strength of interactions in real biological systems than affinity. Therefore, the avidity of an antigen–antibody reaction is dependent on the valencies of both antigens and antibodies and is greater than the sum total of individual affinities.
The term specificity refers to the ability of an individual antibody-combining site to react with only one antigenic determinant or the ability of a population of antibody molecules to react with only one antigen. Antigen–antibody reactions usually show a high degree of specificity.
Despite this, cross-reactions between antigens and antibodies, however, do occur and are sometimes responsible for causing diseases in hosts and for causing false results in diagnostic tests.
Although antigen–antibody reactions are highly specific, in some cases antibody elicited by one antigen can cross-react with an unrelated antigen. Such cross-reactivity occurs if two different antigens share an identical or very similar epitope. In the latter case, the antibody’s affinity for the cross-reacting epitope is usually less than that for the original epitope. Antisera containing polyclonal antibodies can often be found to cross-react with immunogens partially related to those used for immunization, due to the existence of common epitopes or of epitopes with similar configurations.