Antigens, Haptens and Adjuvants

Antigens, Haptens and Adjuvants

1. Dr. P. Saranraj M.Sc., M.Phil., Ph.D., NET
Assistant Professor
Department of Microbiology
Sacred Heart College (Autonomous)
Tirupattur, Vellore District.
Mobile: [phone]
E.mail: [email protected]
ANTIGENS, HAPTENS
AND ADJUVANTS

2. ANTIGENS
An Antigen is a molecule capable of inducing
an immune response (to produce an antibody) in
the host organism.
Most antigens are either Proteins or Polysaccharides.
Lipids and Nucleic acids are usually antigenic only when
combined with Proteins and Polysaccharides.
Most antigens are large, complex molecules with a
molecular weight generally greater than 10,000 to 6,00,000
Daltons.
The ability of a molecule to function as an antigen depends
on its Size, Structural complexity, Chemical nature
and Degree of foreignness to the host.

3. The Antigen may originate from within the body (Self –
antigen or Autoantigens) or from the external environment
(Non-self antigen or Heteroantigens or Foreign antigen).
Self - antigens: Originate within the human body during
Autoimmune diseases.
Non - self antigens: Microorganisms, Snake venom, Certain
proteins from foods and Components of Serum & RBCs.  
Antigenic compounds are often components of invading
microbes, such as capsules, cell walls, flagella, fimbriae, and
toxins of bacteria; the coats of viruses; or the surfaces of
other types of microbes.
Non-microbial antigens include Pollen, Egg white, Blood
cell surface molecules, Serum proteins, and Surface
molecules of transplanted tissues & organs.

4. HAPTENS
Hapten – Derived from Greek word “Haptein” which means
“To fasten”.
Many low molecular weight organic molecules that are not
antigenic by themselves but become antigenic if they bond to
a larger carrier molecule such as a protein. These low
molecular weight antigens are called Haptens.
Example of a Hapten is Penicillin, a small molecule and is
not antigenic. However, when it is combined with certain
serum proteins of sensitive individuals, the resulting
molecule becomes immunogenic, activates lymphocytes,
and initiates a severe and sometimes fatal Allergic reaction (a
type of immune response).

5. The term Hapten was first coined by Karl Landsteiner.
Haptens may be Complex or Simple.
Complex Haptens
Polyvalent
Precipitate with Specific antibodies.
Simple Haptens
Univalent
Non – precipitate with Specific antibodies.

6. Examples of Haptens
1)Aniline (an Organic compound) and its derivatives (o-, m,
p-benzoic acid) (First researched Hapten).
2)Urushiol present in Toxicodendron radicans, Poison ivy plant.
3)Hydralzine, a blood pressure lowering drug.
4)Fluorescein, a fluorescent dye.
5)Biotin, a Vitamin – B7 (Formerly known as Vitamin – H or
Coenzyme – R).
6)Penicillin, an Antibiotic.
7)Digoxigenis, a steroid found in the flowers & leaves of the
plants Digitalis purpurea, Digitalis orientalis & Digitalis
lanata.
8)2,4 – Dinitrophenol, an Organic compound.

7. Similarities - Antigens & Haptens
1)Both are Antigenic.
2)Both are present on external cellular surfaces of
microbial pathogens and other agents.
3)Both form a part of the defense mechanism system
between antigen and antibody.
4)Both have the ability to bind to the antibody via
Weak linkages such as Ionic interactions, Hydrogen
bonding and Hydrophobic interactions.

8. Differences - Antigens and Haptens
ANTIGENS HAPTENS
An antigen is a foreign body,
which has the ability to trigger
the host immune system to
produce an immune reaction
by binding to an antibody.
A Hapten is an incomplete
antigen which is not originally
immunogenic.
Antigen directly binds to the
antibodies produced and
initiate an immune reaction.
Hapten binds to an antibody
but does not have the ability to
trigger the host immune
system to produce an immune
reaction.

9. Differences - Antigens and Haptens
ANTIGENS HAPTENS
Antigens are not conjugating
with a carrier molecule.
Haptens conjugate with carrier
molecules.
Antigens are used in in
vitro techniques such as ELISA
and in Pharmacological
purposes.
Haptens are used in
Antibiotics and Anesthetics
designing.
Antigen reactions are
Antigenic and Immunogenic.
Hapten reactions are only
Immunogenic.

10. ADJUVANTS
Adjuvants are substances that, when mixed with an antigen
and injected with it, enhance the immunogenicity of that
antigen.
The word Adjuvants was derived from the Latin word
“Adjuvare” which means “To help”).
Adjuvants are often used to boost the immune response
when an antigen has low immunogenicity or when only
small amounts of an antigen are available.
Adjuvants may be added to a Vaccine to modify the Immune
response.
The antibody response of Mice to immunization with Bovine
serum albumin (BSA) can be increased fivefold or more if
the BSA is administered with an adjuvant.

11. Commonly used Adjuvants
1)Aluminium potassium sulfate (Alum) (First Aluminium salt used as
Adjuvant and now completely replaced by aluminium hydroxide and
aluminium phosphate for commercial vaccines).
2)Freund’s Incomplete Adjuvant (Oil in water emulsions).
3)Freund’s complete Adjuvant (Inactivated and dried Mycobacterium
tuberculosis).
4)Plant Saponins.
5)Cytokines
6)Quil A (Deteregent)
7)Mineral oil and Food based oil (Peanut oil)
8)Squalene (Carbon organic compound obtained from Shark liver oil)
9)Bacterial Products – Toxoids, Lipopolysaccharides and Killed
Bordetella pertussis.
10)Synthetic Polynucleotide.

12. Mechanism of Immune stimulation by
Adjuvants
Extend the presence of antigen in the blood.
Helps the Antigen presenting cells absorb antigen.
Activate Macrophages and Lymphocytes.
Support the production of Cytokines.

13. IMMUNOGENICITY AND
ANTIGENICITY
Immunogenicity - Ability to induce a Humoral or Cell
mediated immune response
Antigenicity - Ability to combine specifically with the
final products of the Immune response (i.e. Secreted
antibodies or Surface receptors on T-cells).

14. DETERMINANTS OF ANTIGENS (OR)
ANTIGENICITY
Some of the Determinants of Antigenicity or Antigens are:
(i) Molecular Size, (ii) Chemical nature, (iii) Foriegnness,
(iv) Physical form, (v) Antigenic specificity, (vi) Species
specificity, (vii) Organ specificity, (viii) Isospecificity (ix)
Autospecificity, (x) Genetic factors, (xi) Age, (xii)
Degradability, (xiii) Dose of Antigen, (xiv) Route of
Administration and (xv) Adjuvants

15. (i) Molecular Weight
Antigenicity is related to Molecular weight.
Very large molecules (Hemocyanins, Tetanus toxoid, Egg
albumin and Thyroglobulin) (10000 Da – 600000 Da) are
highly antigenic.
Low molecular weight molecules (Insulin) (less than
5000 Da) are non-antigenic.
Many low molecular weight organic molecules that are not
antigenic by themselves but become antigenic if they bond to
a larger carrier molecule such as a protein. These low
molecular weight antigens are called Haptens.

16. (ii) Chemical Nature
Antigens are mostly made up of Proteins and some are
Polysaccharides.
Proteins which are composed of about 20 different Amino
acids are better Antigens than Polysaccharides which have
only four or five Monosaccharide units.
All the Proteins are not Antigenic in nature (E.g., Gelatin).
Lipids and Nucleic acids are usually non-antigenic but
they become antigenic when combined with Proteins and
Polysaccharides.
Presence of an Aromatic radical is essential for rigidity
and antigenicity of a substance.

17. (iii) Foreignness
An antigen must be a foreign substances (non-self) to
the animal to induce an immune response.
The Antigenicity of a substance is related to Foreignness.

(iv) Physical Form
Particulate antigens are more immunogenic than
Soluble antigens.
Denatured antigens are more immunogenic than the
Native form.

18. (v) Antigenic Specificity
The basis of Antigenic specificity is Stereochemical
(spatial arrangement of atoms and groups in molecules).
The Antigenic specificity was first demonstrated by
Obermayer & Pick and confirmed by Karl
Landsteiner.
Antigenic specificity is not absolute. Cross reaction can
occur between antigens which bear Stereochemical
similarities.
Antigen Specificity depends on the specific actives sites
on the antigenic molecules (Antigenic determinants).

19. Antigenic determinants or Epitopes are the part of an
Antigen that is recognized by the Immune system, B – cells
or T – cells.
The Epitopes are divided into two categories:
1)Conformational Epitopes - Composed of discontinuous
sections of the antigen’s amino acid sequence.
2)Linear Epitopes - Formed by a continuous sequence of
amino acids from the antigen.

20. (vi) Species Specificity
Tissues of all individuals in a particular species possess,
Species - specific antigen.
Human Blood proteins can be differentiated from Animal
protein by Specific antigen-antibody reaction.

(vii) Organ Specificity
Organ specific antigens are confined to particular
organ or tissue.
Certain proteins of Brain, Kidney, Thyroglobulin and Lens
protein of one species share specificity with that of
another species.

21. (viii) Isospecificity
Isoantigens are antigens found in some species but not
in all member of species (E.g., Human erythrocyte
antigen based on which individuals can be classified into
different Blood groups).
 Isoantigens are of clinical importance in Blood
Transfusion and during Pregnancy.
(ix) Autospecificity
The Autologous or Self - antigens are ordinarily not
Immunogenic, but under certain circumstances Lens
protein, Thyroglobulin and others may act as an
Autoantigens.

22. (x) Genetic Factors
Some substances are Antigenic or Immunogenic in one
species but not in another. Similarly, some substances
are Antigenic or Immunogenic in one individual but not
in others.
The species or individuals may lack or have altered genes
that code for the receptors for antigen on B cells and T cells.

(xi) Age
Age can also influence Antigenicity or Immunogenic.
Usually the very young and the very old have a
diminished ability to elicit and immune response in
response to an Immunogen.

23. (xii) Degradability
Antigens (T – dependent antigens) that are easily
phagocytosed are generally more immunogenic because the
development of an immune response requires that the
antigen be phagocytosed, processed and presented to Helper
T - cells by an Antigen presenting cell (APC).

(xiii) Dose of Antigen
The dose of administration of an antigen can influence its
immunogenicity.
Dose of antigen above or below which the immune response
will not be optimal.

24. (xiv) Route of Administration
Generally the subcutaneous route is better than the
intravenous or intragastric routes.
The route of antigen administration can also alter the nature
of the response.
Antigen administered intravenously is carried first to the
spleen, whereas antigen administered subcutaneously moves
first to local lymph nodes.

25. (xv) Adjuvants
Adjuvants are substances that, when mixed with an antigen
and injected with it, enhance the immunogenicity of that
antigen.
Adjuvants are often used to boost the immune response
when an antigen has low immunogenicity or when only
small amounts of an antigen are available.
Adjuvants may be added to a Vaccine to modify the Immune
response.