Antigen Antibody Reaction

The interactions between antigens and antibodies are known as antigen–antibody reactions. The reactions are highly specific, and an antigen reacts only with antibodies produced by itself or with closely related antigens.


Antibodies recognize molecular shapes (epitopes) on antigens. Generally better the fit of the epitope (in terms of geometry and chemical character) to antibody combining site,  more favorable the interactions that will be formed between antibody and antigen and  higher the affinity of  antibody for antigen. The affinity of  antibody for  antigen is one of the most important factors in determining antibody efficacy in vivo.


The antigen- antibody interaction is bimolecular irreversible association between antigen and antibody. The association between antigen and antibody includes various non-covalent interactions between epitope (antigenic determinant) and variable region (VH/VL) domain of antibody.



Strength of Ag-Ab interactions

Affinity- Strength of the reaction between a single antigenic determinant and a single combining site on the antibody.

Combined strength of total non-covalent interactions between single Ag- binding site of Ab and single epitope is affinity of Ab for that epitope.

It is also called as association constant for binding (KA)

KA= k1/k2 

Low affinity Ab: Bind Ag weakly and dissociates readily.

High affinity Ab: Bind Ag tightly and remain bound longer.



Avidity: Collective affinity of multiple binding sites (Affinity+ Valence)

Note Point: Valence= Number of epitopes

Strength of multiple interactions between multivalent Ab and Ag is avidity. Avidity is better measure of binding capacity of antibody than affinity. High avidity can compensate low affinity.


Cross reactivity

Antibody elicited by one Ag can cross react with unrelated Ag if they share identical epitope or have similar chemical properties.

  • The ability of an individual Ab combining site to react with more than one antigenic determinant.
  • The ability of a population of Ab molecules to react with more than one Ag.




The ability of an individual antibody combining site to react with only one antigenic determinant or a population of antibody molecules to react with only one antigen.


Types of Ag-Ab reactions:

  1. Agglutination
  2. Precipitation
  3. Complement Fixation
  4. Enzyme Linked Immunosorbent Assay (ELISA)
  5. Radio Immuno Assay (RIA)
  6. Western Blotting