Ag-Ab reactions Microbiology

Antigen antibody reaction
• What is Antigen?
• A toxin or other foreign substance which induces an immune
response in the body, especially the production of antibodies.
• In immunology, Antigen is a molecule capable of inducing an
immune response on the part of the host organism.

Antigen antibody reaction
• What is Antibody?
• It is a large, Y-shaped protein
molecule produced mainly by plasma
cells that is used by the immune
system to neutralize pathogens such
as pathogenic bacteria and viruses.
• It is also known as immunoglobulin
presented as (lg).
• Epitopes are small chemical groups on
the antigen molecule that elicit and
react with antibody.

Antigen antibody interaction
Types of Ag-Ab reactions:
1. Agglutination reaction:
2. Precipitation and Flocculation reaction:
3. Complement Fixation
4. Opsonization

Agglutination:
An Antigen-Antibody reaction in which antibodies cross link
particulate antigens resulting in a visible clumping of particles is
called Agglutination reaction.
The word agglutination comes from the Latin word
agglutinare (gluing to).
When antibodies are mixed with their corresponding antigens on
the surface of large, easily sedimented particles such as animal
cells, erythrocytes, or bacteria, the antibodies cross-link the
particles, forming visible clumps. This reaction is termed
agglutination.

Mechanism Of Agglutination rxn:
Ab + Ag  Agglutinates.
Antibodies binds to more than one antigens
at a time and brought them near to one
another making clumps.
Imminoglobin M (IgM) plays vital role in
Agglutination. IgM is the largest antibody,
and it is the first antibody to appear in
the response to initial exposure to
an antigen.

Agglutination by IgM

Agglutination reactions
Examples are:
Widal Test for Typhoid fever.
Brucella agglutination test for Brucella.
Weil Felix Test for Ricketsiosis.(Rickettsia)

Applications:
Most importantly agglutination tests are performed routinely by
blood banks to determine ABO and Rh blood groups in preparation
for transfusion.
It is used for the detection of both antibodies and antigens in
serum and other body fluids.
Used for detection of various Diseases.

Precipitation/Floculating Reaction:
Such a reaction in which when an antibody binds to a soluble
antigen, the antigen becomes insoluble and is precipitated which
may settle at the bottom of Test tube or may float.
If the complex formed floats then it is flocculating reaction and if
it is precipitated then it is precipitation reaction.

Mechanism of Precipitation/Floculating Rxn:
A single antibody can bind to more than one antigen if it has got
greater velency. For example IgM has got velency = 10, so it can
bind to 10 antigens if it gets suitable epitopes.
The amount of precipitation depends upon the quantities of both
Ag and Ab.
Following graph explains the claim.

Precipitation Reaction:
The precipitation test can be done either in solution or in solid
medium.
Precipitation in soln:
It is used clinically to measure the amount of immunoglobulins
like IgM, IgG etc in blood plasma.
Precipitation in Agar:
This is done either as single or double diffusion.

Precipitation Reaction:

Complement Fixation:
The complement system is a system of serum proteins that react
with antigen-antibody complexes.
Some times the Ab’s are not able to dominate Ag’s, so inactive
enzymes of compliment system are activated in cascade manner in
the presence of Ag-Ab complexes.
Each protein component must be activated sequentially under
appropriate conditions for reaction to progress.
This reaction occurs on a cell surface of antigen, it will result in
the formation of trans-membrane pores and therefore destruction
of the cell.

The Compliment Fixation assay (CFA), can be used to check the
presence of specific Ag or specific Ab in blood serum of an
individual.
The reaction consist of following two steps:
I. Ag and Ab one known and one unknown are mixed up and
measured amount of compliment is added. If the Ag and Ab
match they will combine and use up Comlement.
II. An indicator system containing RBC’s & Anti RBC’s antibodies is
added now.
C1>C4b>C2b>C3b>C5b>6789

Results:
If the antibody matched antigen in 1st step and complement is
utilized there will be no complement unbound and no hemolysis
will occurs.(Positive test result)
If the antibody did not matched the antigen in 1st step, then
complement will be free to attach to RBCs and bring about
hemolysis.(Negative test result)

Opsonization & Phagocytosis:
The process by which bacteria and other cells are altered in such
a manner that they are more readily and more
efficiently engulfed by phagocytes.
It can also be defined as: The addition of opsonin to a bacterium,
to insure that the microorganism is radially identified, and more
efficiently taken up by macrophage.
Opsonins are light sensitive proteins and has five major classes
having different functions to perform.

Process of opsonization:
Opsonization involves the binding of an opsonin, e.g., antibody, to
an epitope on an antigen.
 After opsonin binds to the membrane, phagocytes are attracted to
the pathogen.
The Fab portion of the antibody binds to the antigen, whereas the
Fc portion of the antibody binds to an Fc receptor on the phagocyte,
facilitating phagocytosis.
The core receptor & opsonin complex also creates byproducts
like C3b and C4b which are important components for the efficient
function of the complement system. These components are
deposited on the cell surface of the pathogen and aid in its
destruction.

Ag-Ab reactions Microbiology

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