Antigens are substances that induce detectable immune responses when introduced to an animal host. They bind specifically to antibodies or T-cell receptors. Immunogens are antigens that generate an immune response, while not all antigens are immunogenic. Factors like molecular size, complexity, number of epitopes, dosage, and route of administration affect a substance's immunogenicity. Epitopes are the specific regions on antigens that trigger immune reactions. Haptens are small molecules that become immunogenic when attached to larger carrier molecules. Adjuvants are substances that enhance immune responses to antigens when co-administered.

1. ANTIGENS
Antigens are substances which are able to induce
detectable immune responses when introduced into an
animal host. Immune responses could be cellular or
humoral
Dr.jassim Abdo
Immunology Lecture

2. Antigen Recognition by the Specific
Immune System:
Recognition by B-cell and T-cell Receptors
Antigen (Ag): Binds specifically to an antibody
binding site (Ab), or to a T-cell receptor (TCR)
Immunogen: Binds specifically to an antibody
binding site or to a T-cell receptor,
and generates a humoral or cellular immune
response.
All immunogens must be antigens,
Not all antigens can generate a response.

3. Method of antigen
administration
• Immune response may differ according
to the route of administration
• Level of immune response is dose-
dependent
• Excessively high dose may induce a
state of specific unresponsiveness

4. Factors Affecting Immunogenicity
1. Foreignness (Difference from "Self")
In general, molecules must be recognized as "nonself.“
2. Molecular Size
Generally, molecules with a molecular weight less than
10,000 are weakly immunogenic, and very small ones
(eg, amino acids) are nonimmunogenic.
The potent immunogens are large proteins. Certain
small molecules (eg, haptens) become immunogenic
only when linked to a carrier protein.

5. 3. Chemical and Structural Complexity
A certain amount of chemical complexity is
required—for example, amino acid homopolymers
are less immunogenic than heteropolymers
containing two or three different amino acids.
4. Antigenic Determinants (Epitopes)
The smallest unit of a complex antigen that is
capable of binding to an antibody is known as an
antigenic determinant, or epitope. In general, a
determinant is roughly five amino acids or sugars in
size.

6. 5. Genetic Constitution of the Host
Two strains of the same species of animal may respond differently
to the same antigen because of a different composition of genes
involved in the immune response, eg, different MHC alleles.
6. Dosage, Route, and Timing of Antigen Administration
Adjuvants is substance that enhance the immune response to an
immunogen. For example, by facilitating uptake into antigen-
presenting cells.

7. EPITOPES
• Most foreign particles are composed of
complex mixture of proteins, polysaccharides,
lipopolysaccharides, lipids and nucleoproteins
• Such large molecules have specific regions
responsible for antigenicity
• Epitopes are regions on the surface of
molecules that specifically trigger immune
reactions
• Epitopes are also called antigenic determinants

8. • Epitopes are also called antigenic
determinants
• An antigen may possess more than one
antigenic determinant
• The antigenic determinants on an antigen
vary in immunogenicity
• Animal host respond better to an
immunodominant epitope on an antigen
• An antigen may possess similar epitopes to
those present on the host’s self antigen

9. However, the cell of the immune system only recognize and
respond to foreign epitopes
• The number of epitopes on an antigen is related to its
size
• Usually about one epitope is present for each five kDa of
protein
• Immunopotency describes the capacity of a region of an
antigen molecule to serve as an
• antigenic determinant and induce the formation of specific
antibody

10. Immunopotency is
determined by:
• Accesibility: exposure to the aqueous
environment
• Charge: electrical charges are dominant
factor in specificity
• Genetic factor: ability to induce
immune response is under genetic
control

11. HAPTENS
• Small molecules (e.g. drugs, hormones), or chemical groups with molecular
wight of less than 1000Da which when bound to other larger molecules can
function as epitopes
• Haptens are too small to be appropriately processed and presented to the
immune system and are therefore not antigenic
• When haptens are linked to a larger molecule, a new epitope is formed on
the larger molecule
• When this is injected into an animal host, immune response develops with
antibody formation

12. • The antibody can react with the hapten in the
larger molecule
• Haptens are non-immunogenic substances but
can react with antibody in a specific manner
• Antigens are capable of inducing cellular
immunity mediated by T-lymphocytes but haptens
are unable to do so.
• The reactions of drugs which serve as haptens
with body proteins may lead to allergies
• Examples of haptens: dinitrophenols, penicillin

13. ADJUVANTS
• Substances that enhance the immune response to an
antigen when administered along with that particular
antigen.
Mechanism of action:
• Depot adjuvants: serve to protect antigen from rapid degradation and thereby
prolong immune responses
• Particulate adjuvants: effectively deliver antigens to antigen presenting cells,
enhance cytokine production by antigen presenting cells, enhance T-helper cell
responses and enhance cell mediated immunity
• Immunostimulatory adjuvants: enhance cytokines production, T-helper cell
response and enhance cell mediated immunity

14. Examples:
• Depot adjuvants: Aluminium phosphate, Aluminium hydroxide,
Treund’s incomplete adjuvants (water-in-oil emulsion)
• Particulate adjuvants: liposomes, microparticles, immunestimulatory
complex
• Immunostimulatory a djuvants: glucose, dextran sulphate,
detergents, saponins, lipopolysaccharides, anaerobic corynebacteria,
bacillus calmette-Guerin (BCG m. boris), Borditella pertussis etc.
• Mixed adjuvant: treund’s complete adjuvant (water-in-oil emulsion
plus mycobacterium)