An immunogen refers to a molecule that is capable of eliciting an immune response, whereas an antigen refers to a molecule that is capable of binding to the product of that immune response (Ab). So, an immunogen is necessarily an antigen, but an antigen may not necessarily be an immunogen The terms immunogen and antigen are often used interchangeably but the later is more common. Antibodies are Globulin Protein (Immunoglobulin) that are synthesized in the Serum and Tissue fluids. It reacts specifically with the antigen that stimulated their production. There are two types serum proteins: albumin and globulin There are Three types of globulins . 1. Alpha globulin 2. Beta globulin 3. Gamma globulin (Antibodies) Gamma globulins are responsible for immunity. So they are called as Immunoglobulin (Ig) The binding of an antibody with an antigen of the type that stimulated the formation of antibody that results in the following reaction  Agglutination  Precipitation  Complement fixation  Phagocytosis  Neutralization of an exotoxin  Opsonization  Tissue fixation  Chemotaxis  Activation of mast cells and basophils PPT prepared by: DR.PRINCE C P Associate Professor , Department of Microbiology, Mother Theresa Post Graduate & Research Institute of Health Sciences (Government of Puducherry Institution)

1. ANTIGEN
DR.PRINCE C P
Associate Professor , Department of
Microbiology,
Mother Theresa Post Graduate & Research Institute of Health
Sciences
(Government of Puducherry Institution)

2. Antigens vs Immunogen
 An immunogen refers to a molecule that is
capable of eliciting an immune response,
whereas an antigen refers to a molecule that is
capable of binding to the product of that
immune response (Ab).
 So, an immunogen is necessarily an antigen,
but an antigen may not necessarily be an
immunogen
 The terms immunogen and antigen are often
used interchangeably but the later is more
common.

3. Foreign body
Foreignness: An antigen must be foreign or
alien to the host with which it makes contact.
The greater the phylogenetic difference, the
more Foreign something becomes.

4. Haptens
 Haptens are partial antigens. That is:
 a.Haptens are antigenic: they can react with
immune lymphocytes or antibodies.
 b. However, haptens are not immunogenic:
they can not by themselves cause the
production of immune lymphocytes or
antibodies.
 Haptens are usually molecules which are too
small to be immunogenic.
 Hapten + carrier = complete antigen
(immunogens)

5. Properties of Antigen
 1. Antigenic Determinant
 2. Foreignness
 3. Complexity
 4. Molecular Size
 5. Charge
 6. Solubility
 7. Accessibility
 8. Valency

6. Types antigens
 a. Autologous antigens are found within the
same individual; that is, they are not foreign to that
individual. For example, a skin graft from an
individual's thigh to his chest is an auto graft, and
is not foreign.
 b. Syngeneic antigens are found in genetically
identical individuals (e.g., individuals from an
inbred strain of mice of identical twins). A graft
between members of an inbred strain is a
syngeneic graft or an isograft, and is not foreign.
 c. Allogeneic antigens (alloantigens) are found
in genetically dissimilar members of the same
species. For example, a kidney transplant from
mother to daughter is called an allograft or a

7. Types of antigens
 TD-Ag (T-cell dependent Ag ) TD-Ag
stimulate B cell to produce Ab with the help of
T cell
 TI-Ag (T-cell independent Ag) TI-Ag can
stimulate B cells to produce Ab without the
help of T cell

8. ANTIBODY
DR.PRINCE C P
Associate Professor , Department of
Microbiology,
Mother Theresa Post Graduate & Research Institute of Health
Sciences
(Government of Puducherry Institution)

9. Immunoglobulin
 Antibodies are Globulin Protein
(Immunoglobulin) that are synthesized in the
Serum and Tissue fluids.
 It reacts specifically with the antigen that
stimulated their production.
 There are two types serum proteins: albumin and
globulin
 There are Three types of globulins .
 1. Alpha globulin
 2. Beta globulin
 3. Gamma globulin (Antibodies)
 Gamma globulins are responsible for immunity. So

10. Functions of the Antibody
 Confer protection against microbial
pathogens by:
 They prevent the attachment of microbes to
mucosal surface of the host.
 They reduce the virulence of microbes by
neutralizing the toxins and viruses.
 They facilitate the phagocytosis by
opsonization of microbes
 They activate complement, leading to
complement mediated activities against
microbes

11. Other gamma globulins
 The immunoglobulins not only includes the
antibody globulins but also the,:
 Cryoglobulins
 Macroglobulins
 Abnormal myeloma proteins
 Immunoglobulins are immunologically active
serum proteins

15. Structure of Ig
 Immunoglobulin is a glycoprotein.
 It is an Y or T shaped molecule.
 It is made up of 4 polypeptide chain.
 Of these, 2 chains are short chains, also
called as Light chains. (L – Chain) they are
identical.
 The other two are longer chains, called as
Heavy chain. (H - Chain) they are also
identical.
 Each light chain is made of 214 Amino Acids.
Each heavy chain is made up of 450-700
aminoacids

17. Structure of Ig
 The immunoglobulin consists of two regions,
namely
 Variable region (V - Region)
 Constant region (C - region).
 In the constant region, the amino acid sequence
remains constant in most of the
Immunoglobulins. In the variable region, the
amino acid sequence shows variability.
 Based on the function aspect, two regions can be
recognized in the immunoglobulin. Fab( Fraction
antigen binding portion) & Fc ( fraction constant
region)

18. PROPERTIES OF
IMMUNOGLOBULINS
 They can agglutinate antigens.
 They form precipitate with antigens.
 They can cross placenta (IgG).
 They can have reaginic activity (IgE).
 They are involved in complement fixation
(IgG & IgM).
 They fix macrophages (IgG).
 They fix mast and basophils cells (IgG).

21. Ig G
 Gamma Heavy chain
 In its four forms, provides the majority of
antibody-based immunity against invading
pathogens.The only antibody capable of
crossing the placenta to give passive immunity
to the fetus.
 Formed during chronic stage of infection

22. Ig M
 Mu Heavy chain
 Expressed on the surface of B cells
(monomer) and in a secreted form (pentamer)
with very high avidity.
 Eliminates pathogens in the early stages of B
cell-mediated (humoral) immunity before there
is sufficient IgG
 Formed during Acute stage of infection

23. Ig A
 Alpha Heavy chain
 Found in mucosal areas, such as
the gut, respiratory tract and urogenital tract,
and prevents colonization by pathogens. Also
found in saliva, tears, and breast milk.
 Secretory antibody

24. Ig D
 Delta Heavy chain
 Functions mainly as an antigen receptor on B
cells that have not been exposed to
antigens. It has been shown to
activate basophils and mast cells to
produce antimicrobial factors.

25. Ig E
 Epsilon Heavy chain
 Binds to allergens and
triggers histamine release from mast
cells and basophils, and is involved in allergy.
Also protects against parasitic worms.

27. Ag-Ab Reactions
( in-vitro Ag-Ab reactions are Known as Serlogical
Reactions)
 The binding of an antibody with an antigen of the type
that stimulated the formation of antibody that results in
the following reaction
 Agglutination
 Precipitation
 Complement fixation
 Phagocytosis
 Neutralization of an exotoxin
 Opsonization
 Tissue fixation
 Chemotaxis
 Activation of mast cells and basophils

28. FACTORS THAT AFFECT ANTIGEN-ANTIBODY
REACTION
 pH
 Salt concentration
 Temperature
 Concentration of antigen and antibody
 Affinity and avidity of the antibody

29. SEROLOGICAL TESTS
 Serology refers to using antigen-antibody
reactions in the laboratory for diagnostic
purposes. Its name comes from the fact
that serum, the liquid portion of the blood
where antibodies are found is used in
testing.

30. SEROLOGICAL TESTS
 TITER
 The minimum volume of a solution needed
to reach the end point in a titration.
 The concentration of an antibody, as
determined by finding the highest dilution at
which it is still able to cause agglutination of
the antigen.

31. SEROLOGICAL TESTS
 Serologic testing may be used in the clinical
laboratory in two distinct ways:
 a. To identify unknown antigens (such as
microorganisms). This is called direct serologic
testing. Direct serologic testing uses a
preparation known antibodies,
called antiserum, to identify an unknown
antigen such as a microorganism.
 b. To detect antibodies being made against a
specific antigen in the patient's serum. This is
called indirect serologic testing. Indirect serologic
testing is the procedure by which antibodies in a
person's serum being made by that individual
against an antigen associated with a particular
disease are detected using a known antigen.

32. SEROLOGICAL TESTS.
 a. Agglutination: Known antiserum causes
bacteria or other particulate antigens to clump
together or agglutinate. Molecular-sized
antigens can be detected by attaching the
known antibodies to larger, insoluble particles
such as latex particles or red blood cells in
order to make the agglutination visible to the
naked eye.
 b. Precipitation: Known antiserum is mixed
with soluble test antigen and a cloudy
precipitate forms at the zone of optimum
antigen-antibody proportion.

33. APPLICATION OF AGGLUTINATION
TEST
 Blood Typing
 Rh Typing
 Coomb’s Test
 Diagnosis of bacterial infection. Ex: typhoid
fever (Widal test), Brucellosis, leptopspirosis
 Diagnosis of viral infections
(Haemagglutination) Ex: Diarrhea caused by
Rota virus.
 Diagnosis of protozoal infection. Ex:
Toxoplasmosis.
 Diagnosis of some autoimmune diseases. Ex:
Rheumatoid factor, Systemic Lupus

34. SEROLOGICAL TESTS
 c. Complement-fixation: Known antiserum is
mixed with the test antigen and complement is
added. Sheep red blood cells and hemolysins
(antibodies that lyse the sheep red blood cells
in the presence of free complement) are then
added. If the complement is tied up in the first
antigen-antibody reaction, it will not be
available for the sheep red blood cell-
hemolysin reaction and there will be no
hemolysis. A negative test would result in
hemolysis.

36. SEROLOGICAL TESTS
 d. Enzyme-linked immunosorbant assay or
ELISA (also known as Enzyme immunoassay
or EIA):Test antigens from specimens are passed
through a tube (or a membrane) coated with the
corresponding specific known antibodies and
become trapped on the walls of the tube (or on
the membrane). Known antibodies to which an
enzyme has been chemically attached are then
passed through the tube (or membrane) where
they combine with the trapped antigens. Substrate
for the attached enzyme is then added and the
amount of antigen-antibody complex formed is
proportional to the amount of enzyme-substrate
reaction as indicated by a color change.

40. SEROLOGICAL TESTS
 e. Radioactive binding techniques: Test antigens
from specimens are passed through a tube coated
with the corresponding specific known antibodies and
become trapped on the walls of the tube. Known
antibodies to which a radioactive isotope has been
chemically attached are then passed through the tube
where they combine with the trapped antigens. The
amount of antigen-antibody complex formed is
proportional to the degree of radioactivity.
 f. Fluorescent antibody technique : A fluorescent
dye is chemically attached to the known antibodies.
When the fluorescent antibody reacts with the antigen,
the antigen will fluoresce when viewed with a
fluorescent microscope.

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