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ANTIGEN-ANTIBODY REACTION OR ANTIGENANTIBODY INTERACTION
Antigen-antibody reaction or antigen-antibody interaction is a particular chemical interaction
between antibodies generated by B cells of the white blood cells and antigens during the
immune reaction
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It is the basic biological process that the body uses to defend itself against various foreign
particles like viruses and their toxic chemicals
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The immunological complex
is subsequently transferred to cellular systems where it can be eliminated or deactivated
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While others, known as haptens, require the assistance
of other molecules (carrier proteins) to activate an immunological response
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They could be polysaccharides, lipids, proteins, or peptides
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Antibody (Ab)
An antibody is a component that the immune system produces in response to antigens
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They act together to exhibit an immunological
response
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Paratope is the name of the antigen binding site
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Antigen-Antibody Reaction
Antigens and antibodies combine specifically with each other
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Ag-Ab reaction is a common acronym
for it
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These reactions serve as the foundation for the detection of both specific and non-specific Ags,
such as enzymes that cause non-specific diseases
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Stages of Antigen-Antibody Reaction
There are three stages to the interactions between Ag and Ab
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The second stage results in visible phenomena like agglutination, precipitation, etc
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Properties of Antigen-Antibody Reaction
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Significantly specific reaction
Occurs in a noticeable manner
Non-covalent reactions (Ionic bonds, Van der Waals forces, Hydrophobic interactions,
Hydrogen bonds)
Antibodies and antigens are not denatured
Reversible
Affinity: This refers to how strongly an antigen binds to a certain antigen-binding site
on an antibody
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It represents the Ag-Ab complex’s
total strength
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The antibody’s affinity
2
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How epitopes and paratopes are structurally arranged
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Types of Antigen-Antibody Reaction
The types of antigen-antibody reactions are as follows:
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Precipitation Reaction
Agglutination Reaction
Complement Fixation
Immunofluorescence
ELISA – Enzyme-Linked Immunosorbent Assay

Precipitation Reaction
An insoluble precipitate of Ag-Ab complex is produced when a soluble Ag and its Ab combine
in the presence of an electrolyte (NaCl) at a specific pH and temperature
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The precipitation reaction occurs in both liquid and gel media
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Precipitation
results from the combined reaction of the antigen and antibody
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In the gel system, both Ag and Ab rapidly diffuse in all directions
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Multiple bands develop in complex Ag or Ab preparations
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Agglutination Reaction
The particles are clustered or agglutinated when a certain Ag is combined with its Ab in the
presence of electrolytes at an appropriate temperature and pH
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Agglutinogens are the name for the aggregated particulate antigens
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• Tube Agglutination: This is a common technique for estimating the quantity of Ab
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A control tube is retained that contains no antiserum
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The tube demonstrating the highest agglutination is known as the
titre
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A carrier particle has Ag coated on its surface, making the reaction more sensitive by assisting
in the transformation of a precipitation process into an agglutination reaction
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Sometimes, tanned RBC (polystyrene
coated RBC) might be used
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Every person has the 11 proteins that comprise the complement system
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Complement fixation tests make use of the Ag-Ab
complex’s capacity to fix complement
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This is incubated for 18
hours at 4°C
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Immunofluorescence
Fluorescence is the ability to absorb light rays of a certain wavelength and emit light rays of a
different wavelength
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In 1942, Albert Coons and coworkers demonstrated how labelled dyes could be coupled to
antibodies, allowing for the detection of antigens using these labelled dyes
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As a strong emitter of red fluorescence and an effective light absorber (30 times more effective
than fluorescein), phycoerythrin is often used as a label for immunofluorescence
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When compared to radioimmunoassay (RIA), they are more simple, sensitive, affordable, and
risk-free
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and is capable of detecting the Ab
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The surface
of the microtiter plates is coated with envelope proteins developed using recombinant
technology
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• Sandwich ELISA: This method is used for determining whether there is Ag in a
sample
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Unbound Ag is removed from the wells through washing
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Washing is used to remove
unbound Ab’s, followed by the addition of coloured substrate and colour development
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Competitive ELISA: Competitive ELISA is another variant for estimating antigen
concentrations
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An antigen-coated microtiter well is then filled with the antigen-antibody mixture
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In an indirect ELISA, the addition of a secondary antibody (Ab2),
coated with an enzyme, specific for the primary antibody’s isotype can be utilised to estimate
how much primary antibody is attached to each well
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Specific interactions between the antibodies and antigens result in combination, known as
Antigen-Antibody reaction or Antigen-Antibody interaction
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Exogenous (alien to the host immune system),
endogenous (made by internal bacteria and viruses reproducing inside a host cell), and
autoantigens (produced by the host) are the three basic techniques to describe antigens
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Antigens can be any component of
bacteria or viruses, including the coating, surface protein, toxins, capsule, and cell wall
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They are distributed
throughout the body and have several functions

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