antigen-antibody reaction

Immunology 2011
The interaction between antigen and antibody
 ag - ab reaction
 Antigen-antibody reaction is characterized by
following salient features.
Immune Complex
Specificity of Ag-Ab Reaction
Binding Sties of Antigen and Antibody
Binding Forces of Antigen and Antibody
Bonus Effect
Cross Reaction
When antigen and antibody are brought together, the
antibody binds with the antigen to form a complex
molecule called immune complex or antigenantibody complex
Ag + Ab
Ag-Ab complex
Specificity refers to the discriminate ability of
particular antibody to combine with only one
type of antigen.
An antibody will combine with the antigen
which is the cause for its production.
Epitope (antigenic determinant)- The part of the
antigen which combines with the antibody.
Paratope (antigen binding site) - The Part of the
antibody which combines with the antigen.
Binding between antigen and antibody is due
to three factors namely,
1. Closeness between antigen and antibody
2. Intermolecular forces
3. Affinity of antibody
Avidity refers to the capacity of an antiserum
containing various antibodies to combine with the
whole antigen that stimulated the production of
Avidity is used to denote the overall capacity of
antibodies to combine with multivalent antigen
the antibody not only binds with the antigen but also
the antigens are bridged by a single antibody.
In some cases two antigens may be bridged by a single
Such a binding is weak.
But when two antigens are bridge by two antibodies,
the binding will be strong.
This phenomenon of giving extra- strength to the
antigen-antibody complex by the binding of two
antibodies to two antigen molecules is called bonus
An antiserum raised against a given antigen
may sometimes react with another closely
related antigen - cross reaction
 the antigen which produces the cross reaction
is called cross reactive antigen.
 The cross reaction is due to the presence of
one or more identical antigenic determinants
on the related antigen.
Complement fixation
reaction between a soluble antigen and its
antibody resulting in the formation of
insoluble precipitate.
 The antibody - precipitin.
 due to the formation of Antigen-antibody
 The antigen is multivalent and the antibody is
As each antibody is a bivalent molecule, it can
bridge two multivalent antigen molecule.
This bridging leads to the formation of a lattice
which forms the precipitate.
When antigen and antibody are in optimal
concentration the precipitation is complete and
a large lattice is formed.
reaction where the antibody of serum causes the
cellular antigens to adhere to one another to form
Antibodies - agglutinins
antigens – agglutinogens
When red blood cells are agglutinated, the reaction haemagglutination.
bacterial agglutinations
is the dissolution of a cell.
 RBC is lysed - haemolysis.
 bacterial cell - bacteriolysis.
Mechanism of Cytolysis:
 the antigen-antibody complex activates the complement.
 complement binds to the surface antigen of microbe or cell.
 The compliment fixed on the surface of the cell causes the
disruption of the lipid bilayer of the membrane of the
 As a result, a hole is made on the microbe.
 Through this hole the content of the cell are released and
the cell is lysed.
The binding of complement to antigen –
antibody complex is called complement
 When complement is added to a serum
containing an antigen and its antibody, the
complement is activated and
 immediately it binds to the Antigen-antibody
complex and the complement is said to be
antigen- antibody reaction brought about by
exotoxin and antitoxin.
reaction produces flocculates which do not
sediment but remain dispersed in the medium.
Flocculation is somewhat like precipitation,
but the precipitates will not sediment.
Process by which a particular antigen becomes
more susceptible to phagocytosis by
combination with an opsonin.
 Opsonin is an antibody
 when combines with a particulate antigen,
increases the susceptibility of the antigen to
 In Opsonization - the antibody combines with
the surface antigen of bacteria.
This antigen-antibody complex activates complement
The activated complement is attached to the antigenantibody complex to form an antigen-antibody
complement complex.
the antigen-antibody-complement is adhered to the
phagocytic cells.
The microbe ( antigen) fixed on the phagocytic cell is
killed by phagocytosis or lysis.
When antibodies are mixed with fluorescent dyes such
as fluorescein or rhodamine, they emit radiation.
Phenomenon of emitting radiation by antibodies
labeled with fluorescent dye- immunofluorescence.
The immunofluorescence can be observed by a
fluorescent microscope.
Application of Immunofluorescence
It is used to locate and identify antigen in tissues.
Certain pathogenic bacteria can be identified.
Antibodies directed to cell or tissue antigen can be
Direct method
 the antibody labeled with fluorescent dye is
applied directly on the tissue section.
 The labeled antibody binds with specific
 This can be observed under the fluorescent
 rabies can be diagnosed.

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