Antigen Antibody Reactions

Antigen Antibody Reactions

Antigen Antibody Reactions

Introduction

  • The interactions between antigens and antibodies are known as antigen antibody reactions.
  • The reactions are highly specific, and an antigen reacts only with antibodies.
  • These reactions are essentially specific, they have been used in many diagnostic tests for the detection of either the antigen or the antibody.
  • The antigen and antibody reactions also form the basis of immunity against microbial 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 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:

Physicochemical Properties:

  • 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.
  • 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:

  • Affinity denotes the intensity of attraction between antigen and antibody.
  • Low-affinity antibodies bind antigen weakly and tend to dissociate readily, whereas high-affinity antibodies bind antigen more tightly and remain bound longer.

Avidity:

  • Avidity is a measure of the overall strength of binding of an antigen with many antigenic determinants and multivalent antibodies.
  • 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.

Specificity:

  • 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.

Cross-Reactivity:

  • 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.

Types of Antigen–Antibody Reactions

Serological tests are widely used for detection of either antibodies or antigens for diagnosis of a wide variety of infectious diseases.

These serological tests are also used for diagnosis of autoimmune diseases and in typing of blood and tissues before transplantation.

The following are the examples of antigen–antibody reactions:

  1. precipitation
  2. agglutination
  3. complement dependent serological tests
  4. neutralization test
  5. opsonization
  6. immunofluorescence
  7. enzyme immunoassay
  8. radioimmunoassay
  9. western blotting
  10. chemiluminescence assay
  11. immunoelectronmicroscopic tests

Antigen Antibody Reactions

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