What are the Salient Features of Antigen-Antibody Reactions?


Specificity of Antigen-Antibody (AG-AB) Interaction:

Like lock and key mechanism, interaction between Ag and Ab is highly specific. Specific antibody can distinguish even minute differences in structure, charge, etc. of an antigens epitope and binds only with its specific Ag.

This character of antibody is known as “specificity of antibody”. For example Abs produced against blood group B antigens, bind with B antigens only. They never bind with A, or D blood group antigens.


Since each antigen enclose different number of epitopes, immune system produces different antibodies to suit each epitope of the antigen. Bacterial species and types can be identified by using this specific nature of antigen antibody reactions!

Antibody Affinity:

It is the strength of the reaction between a single antigenic determinant and a single binding site of antibody. Since the binding between antigen and antibody is mediated by non covalent bonds, it operates over a very small distance.

The binding strength depends on distance and compatibility of two substances. Increased distance between the two objects weakens the bonding strength.


The sum of the attractive and repulsive forces, such as electrostatic forces, hydrogen bonding, hydrophobic bonding, Van der Waals force etc. operating between the epitope and paratope also influence the affinity.

An antibody with high affinity binds tightly with its suitable antigen and the bonding remains for a longer time. If the affinity of Ab is low the bonding strength is also low and the bond remains for a very short time.

In simple terms affinity is the equilibrium constant that describes the strength of antigen-antibody reaction. Most antibodies have a high affinity for their specific antigens. The affinity of an antibody and monovalent antigen can be described by the equation.



KI = Association rate constant.

k-I = Dissociation rate constant.

The ratio of kl/k-1 is the equilibrium constant “ka”

Since ka is the equilibrium constant, it can be calculated from the ratio of the molecular concentration of bound Ag-Ab complex to the molar concentrations of unbound antigen and antibody at equilibrium.


Ka [Ag-Ab] (Ab)(Ag)

Ka depends upon KI, k is expressed in liters / moles/ second

k-1 is expressed in liters /sec

Antibody Avidity:


Antibody molecules and antigens are multivalent. This multi valency of Abs and Ags tend to increase chances of interaction and also strength of bonding. The overall strength of an antibody binding to multivalent antigen is known as “avidity”.

For instance, if a multivalent antigen holds repetition of similar epitopes, an antibody having more than one binding site can bind with more than one epitope of the same antigen.

Binding of antibody to an antigen at more than one site increases strength of the bond or avidity proportionately. Avidity is influenced by valency of both antibody and antigen. High avidity of antigen and antibody can compensate low affinity.

Cross Reactivity:

There is a possibility of presence of similar epitopes on different antigens. Hence antibodies produced for a specific antigen can cross react with antigens having similar epitopes.

For example antiserums raised against albumin of hens egg can cross react with the albumin of ducks egg. It is often observed among polysaccharide antigens that contain similar oligosaccharide residues.

Cross reaction is the cause for blood transfusion reactions. In blood transfusion, there is no need of pre sensitization to generate rejection reactions against a mismatched blood.

The blood group antibodies elicited by microbial antigens are cross reactive in nature and can react with blood group antigens during transplantation.

Cross reactivity is responsible for various autoimmune diseases also. Cross reactivity of antibodies, produced against cowpox antigen with variola virus is responsible for the success of Jenner’s vaccination trial against small pox.

Cross reactivity character of Abs has been used differently in various diagnostic tests. For example, Treponema pallidum (syphilis) infection induces production of antibodies that cross-react with cardiolipin – a substance found in cardiac muscle.

Since it is much easier to obtain pure cardiolipin than pure Treponemal antigens, cardiolipin is used in diagnostic test for syphilis (Wassermann test). In the same way antibodies produced against Rickettsia cross-react with antigens from Proteus.

Since the latter are much easier to obtain, they can be used to test for the former.

Wassermann test – It is a blood test to detect syphilis; a complement fixation test is used to detect antibodies to the syphilis organism treponema; a positive reaction indicates the presence of antibodies and therefore syphilis infection.

Antigen Precipitation:

Soluble antigen molecules can be precipitated by antibodies. Antigen interaction enables antibodies to neutralize, opsonize and inactivate toxic substances and pathogens. The soluble antigens that can precipitate Abs are known as “precipitins”.

Even though Ag-Ab complex formation takes place within minutes, visible precipitates arise more slowly and it depends upon the concentration of Ags and Abs. The precipitin reaction is influenced by the number of paratopes of antibody for the given antigen and by the number of epitopes found on antigen molecule.

Antigen with two or more epitopes is essential to produce precipitation reaction in a polyclonal antiserum. If the antigen is allowed to interact with monoclonal antibodies, formation of precipitate is possible only if the antigen express as at least two or more epitopes for the paratopes of monoclonal antibodies.

Precipitation test can be used to check the similarity of soluble antigens.

Bonus Effect:

The antigen with multivalency or multiple binding sites can react with different antibodies simultaneously. If a single antibody holds two antigens simultaneously the strength of the bond is very weak.

But if an antigen is bridged by two or more antibodies at a time the binding strength of Ag-Ab complex increases proportionately. This added strength of Ag- Ab complex is referred as “bonus effect”.

Increased valency of an antigen and presence of polyclonal antibodies enhances the chance of bonus effect. Generally immune system produces different antibodies at a time to suit different epitopes of an antigen.

Presence of antibodies with different paratopes to match with all the epitopes of the antigen enhances the bonding strength of Ag – Ab complex and

Neutralization of Toxins:

Most of the toxins are antigenic in nature, and induce antibodies production. Binding of Abs to the antigens shields the toxic sites of toxin molecules ensuing neutralization of toxin.

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