Antigen - Antibody Interaction: By: Dr. Luna Phukan

TDC 5TH SEM MAJOR: PAPER 5.
ANTIGEN –ANTIBODY
INTERACTION
BY: DR. LUNA PHUKAN
Antigen-antibody interaction, or
antigen-antibody reaction, is a
specific chemical interaction
between antibodies produced by B
cells of the white blood cells and
antigens during immune reaction.
The antigens and antibodies
combine by a process called
agglutination
Antigen-antibody interaction, or antigen-antibody reaction, is a specific
chemical interaction between antibodies produced by B cells of the white
blood cells and antigens during immune reaction. The antigens and antibodies
combine by a process called agglutination. It is the fundamental reaction in
the body by which the body is protected from complex foreign molecules, such
as pathogens and their chemical toxins. In the blood, the antigens are
specifically and with high affinity bound by antibodies to form an antigen-
antibody complex. The immune complex is then transported to cellular
systems where it can be destroyed or deactivated.
The first correct description of the antigen-antibody reaction was given by
Richard J. Goldberg at the University of Wisconsin in 1952. It came to be known
as "Goldberg's theory" (of antigen-antibody reaction)
There are several types of antibodies and antigens, and each antibody is
capable of binding only to a specific antigen. The specificity of the binding is
due to specific chemical constitution of each antibody. The antigenic
determinant or epitope is recognized by the paratope of the antibody, situated
at the variable region of the polypeptide chain.
The variable region in turn has hyper-variable regions which are unique amino
acid sequences in each antibody. Antigens are bound to antibodies through
weak and noncovalent interactions such as electrostatic interactions, hydrogen
bonds, Van der Waals forces, and hydrophobic interactions.
The principles of specificity and cross-reactivity of the antigen-antibody

interaction are useful in clinical laboratory for diagnostic purposes. One basic
application is determination of ABO blood group. It is also used as a molecular
technique for infection with different pathogens, such as HIV, microbes, and
helminth parasites.
Molecular basis
Immunity developed as an individual is exposed to antigens is called adaptive
or acquired immunity, in contrast to immunity developed at birth, which is
innate immunity. Acquired immunity depends upon the interaction between
antigens and a group of proteins called antibodies produced by B cells of the
blood. There are many antibodies and each is specific for a particular type of
antigen. Thus immune response in acquired immunity is due to the precise
binding of antigens to antibody. Only very small area of the antigens and
antibody molecules actually interact through complementary binding sites,
called epitopes in antigens and paratopes in antibody
Antibody structure
In an antibody, the Fab (fragment, antigen-binding) region is formed from the
amino-terminal end of both the light and heavy chains of the immunoglobulin
polypeptide. This region, called the variable (V) domain, is composed of
amino acid sequences that define each type of antibody and their binding
affinity to an antigen. The combined sequence of variable light chain (VL) and
variable heavy chain (VH) creates three hypervariable regions (HV1, HV2, and
HV3). In VL these are roughly from residues 28 to 35, from 49 to 59, and from 92
to 103, respectively. HV3 is the most variable part. Thus these regions are the
paratope, the binding site of an antigen. The rest of the V region between the
hypervariable regions are called framework regions. Each V domain has four
framework domains, namely FR1, FR2, FR3, and FR
Properties
Chemical basis of antigen-antibody interaction
Antibodies bind antigens through weak chemical interactions, and bonding is
essentially non-covalent. Electrostatic interactions, hydrogen bonds, van der Waals
forces, and hydrophobic interactions are all known to be involved depending on
the interaction sites. Non-covalent bonds between antibody and antigen can also
be mediated by interfacial water molecules. Such indirect bonds can contribute to
the phenomenon of cross-reactivity, i.e. the recognition of different but related
antigens by a single antibody

Structural
model of
an
antibody
molecule.
Rounded
portions
indicate
antigen
binding
sites.
Properties
Chemical basis of antigen-antibody interaction
Antibodies bind antigens through weak chemical interactions, and bonding is
essentially non-covalent. Electrostatic interactions, hydrogen bonds, van der
Waals forces, and hydrophobic interactions are all known to be involved
depending on the interaction sites. Non-covalent bonds between antibody and
antigen can also be mediated by interfacial water molecules. Such indirect bonds
can contribute to the phenomenon of cross-reactivity, i.e. the recognition of
different but related antigens by a single antibody.

Affinity of the interaction
Antigen and antibody interact through a high affinity binding much like lock and key. A dynamic
equilibrium exists for the binding. For example, the reaction is a reversible one, and can be
expressed as:
where [Ab] is the antibody concentration and [Ag] is the antigen concentration, either in free
([Ab],[Ag]) or bound ([AbAg]) state.
The equilibrium association constant can therefore be represented as:
However, these equations are applicable only to a single epitope binding, i.e. one antigen on one antibody.
Since the antibody necessarily has two paratopes, and in many circumstances complex binding occurs, the
multiple binding equilibrium can be summed up as:
where, at equilibrium, c is the concentration of free ligand, r represents the ratio
of the concentration of bound ligand to total antibody concentration and n is the
maximum number of binding sites per antibody molecule (the antibody valence).
The overall strength of the binding of an antibody to an antigen is termed its

avidity for that antigen. Since antibodies are bivalent or polyvalent, this is the sum
of the strengths of individual antibody-antigen interactions. The strength of an
individual interaction between a single binding site on an antibody and its target
epitope is termed the affinity of that interaction.
Avidity and affinity can be judged by the dissociation constant for the interactions
they describe. The lower the dissociation constant, the higher the avidity or
affinity, and the stronger the interaction.
Application
Antigen-antibody interaction is used in laboratory techniques for serological test of blood compatibility and
various pathogenic infections. The most basic is ABO blood group determination, which is useful for blood
transfusion. Sophisticated applications include ELISA, enzyme-linked immunospot (Elispot),
immunofluorescence, and immunoelectrophoresis.
Precipitation reaction
Soluble antigens combine with soluble antibodies in presence of an electrolyte at suitable temperature and
pH to form insoluble visible complex. This is called a precipitation reaction. It is used for qualitative and
quantitative determination of both antigen and antibody. It involves the reaction of soluble antigen with
soluble antibodies to form large interlocking aggravated called lattice

It occurs in two distinct stages. Firstly, the antigen and antibody rapidly form antigen-antibody
complexes within few seconds and this is followed by a slower reaction in which the antibody-
antigen complexes forms lattices that precipitate from the solution.
A special ring test is useful for diagnosis of anthrax and determination of adulteration in food.
Agglutination reaction
It acts on antigen-antibody reaction in which the antibodies cross-link
particulate antigens resulting in the visible clumping of the particle.
There are two types, namely active and passive agglutination. They are
used in blood tests for diagnosis of enteric fever.

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TDC 5TH SEM MAJOR: PAPER 5.

The principles of specificity and cross-reactivity of the antigen-antibody

essentially non-covalent. Electrostatic interactions, hydrogen bonds, van der Waals

forces, and hydrophobic interactions are all known to be involved depending on

be mediated by interfacial water molecules. Such indirect bonds can contribute to

the phenomenon of cross-reactivity, i.e. the recognition of different but related

antigens by a single antibody

essentially non-covalent. Electrostatic interactions, hydrogen bonds, van der

Waals forces, and hydrophobic interactions are all known to be involved

depending on the interaction sites. Non-covalent bonds between antibody and

can contribute to the phenomenon of cross-reactivity, i.e. the recognition of

different but related antigens by a single antibody.

The equilibrium association constant can therefore be represented as:

The overall strength of the binding of an antibody to an antigen is termed its

transfusion. Sophisticated applications include ELISA, enzyme-linked immunospot (Elispot),

immunofluorescence, and immunoelectrophoresis.

soluble antibodies to form large interlocking aggravated called lattice

particulate antigens resulting in the visible clumping of the particle.

used in blood tests for diagnosis of enteric fever.

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