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Antigen antibody interactions play important role in immunological assays which help in detection of disease.Such interaction are of various types e.g.Precipitation,Flocculation, Agglutination, Complement fixation, ELISA,RIA, Immunoflourescence,Immunoprecipitation.
Nidhi A Jodhwani
Ag-Ab reaction is bimolecular association similar to
Association between Ag-Ab involves non-covalent
bonds between epitopes and variable region of
This interaction is very specific that it leads to
development of various immunological assays.
Reaction is very specific.(some time cross reactivity
observed due to same antigenic property)
When antigenic part of large molecule react with Ab
,whole molecule is agglutinated.
There is no denaturation of Antigen or Antibody during
Ag-Ab complex is firm but reversible.The strength of
bond depends upon affinity (attraction) between Ag &
Both Ag & Ab are pequired for precipitation reaction.
Precipitation is the reaction in which a soluble antigen
reacts with soluble antibody to give insoluble product or
Soluble antibodies =‘precipitins’
MECHANISM OF PRECIPITATION
Precipitation can be explained by Lattice hypothesis.
This hypothesis was given by Marrack in 1934
This reaction is greatly affected by concentration of
This effect can be explained by precipitatin curve.
In this curve three zones are oberved
1)Zone of antibody excess (Prozone)
2)Zone of equivalence
3)Zone of antigen excess (postzone)
In this zone antigen concentration is very low compared to
antibody concentration as a result no large Antigen-
Antibody complex are formed
No precipitation is observed
• In this zone precipitation is observed
• Large visible Antigen-Antibody
complex are formed.
Zone of equivalence
• In this zone antigen concentration is very
much higher compared to antibody
• As a result Antigen-Antibody
complex are too small to
Application of precipitation reaction
RID-radial immuno diffussion
Basic mechanism is same as same as the precipitation
If insoluble complex remains suspended in solution instead
of sedimentation the reaction is known as ‘flocculation’.
Interaction between antibody and a particulate
antigen results in visible clumping ,is known as
The main difference between agglutination and
precipitation is about the size of antigen.
In agglutination reaction,antigens are insoluble
Application of agglutination reaction
- In this patient’s serum is tested for the presence of Ab
against bacterial antigens.
-If antigens are present on RBC are to be detected ,process is
known as hemagglutination.
e.g. ABO group typing
-In this, soluble antigens are coated on carrier
particles and after addition of antibodies,
agglutination of carrier particles takes place.
• compliment proteins are able to bind
with ag-ab complex & are able to lyse
compliment fixation test
• In this technique, flourescence property
of dyes are used to demonstrate ag-ab
• Commonly used dyes are :
Flourescein isothiocynate-blue green
Lissamine rhodomine-orange red
• Enzyme Linked Immuno Sorbent Assay
• There are dour types of ELISA
• Radio immuno assay
• In thistechniquee, radioactive isotopes
are used to demonstrate ag-ab reactions.
In this ,antibodies are coated
with insoluble inert material
Here secondary antibodies are
coated with microbeads.