Antigen

1. ANTIGENS,HAPTENS AND IMMUNOGENS
PRESENTER: Angela Charles
FACILITATOR: Dr. Doreen Kamori
Venue: Micro Lab
Date: 26th May 2023

2. OUTLINES:
• Antigens
Definition of terms
• Chemical nature of antigens
• Classification of antigens
• Immunogens
• Factors influencing Immunogenicity
• Immunogens and epitopes
• Haptens
• References

3. ANTIGENS
Antigens:
An antigen is a molecule that is recognized by specific antibodies or
the TCR on T cells
 Its ability to bind with antibodies or T-cell is referred to as
antigenicity.
 The foreign substances (antigens) that induce an immune response
possess two properties; immunogenicity and antigenicity.

4. Cont…
Immunogenicity:
Ability of an antigen to induce immune response in body (both
humoral and/or cell mediated).
•B cells + antigen → effector B cells (plasma cell) + memory B cells
•T cells + antigen → effector T cells (helper T cell or cytotoxic T cell)
+ memory T cells

5. Antigenicity:
• Ability of an antigen to combine specifically with final products
antibodies and/or T cell-surface receptors.
Note: Immunogenic substances are always antigenic, whereas
antigens are not necessarily immunogenic (e.g., Autologous
serum proteins).

6. Epitopes (antigenic determinant)
is the actual molecular structure that interacts with a single
antibody molecule or TCR.
 Some epitopes are found inside the antigen (internal), only
expressed after the antigen has been "processed" by a
phagocytic cell. .

7.  Antigens are multivalent; that is, an antigen molecule carries a
number of different epitopes, sometimes hundreds of them, some
specifying antibody "A", others antibody "B", and so forth, the
valence of an antigen is equal to the total number of epitopes the
antigen possesses.
Paratope:
• Specific site of an antibody that reacts with the corresponding
epitope of an antigen

9. T-cell epitopes properties
Refers to protein antigen-derived peptides presented by
MHC molecule on APCs and recognized by T cells.

10. Why knowing epitopes important?
• Understanding disease etiology
• Immune status monitoring
• Developing diagnosis assays
• Designing epitope based vaccines

11. Adjuvants:
The word "adjuvant“- Latin word adiuvare, meaning to help or aid.
Substances which enhances the body’s immune response to an
antigen
 They are not immunogenic alone but enhance immunogenicity of
an added immunogen.
 When mixed with an antigen and injected with it enhance the
immunogenicity of that antigen.
 These substances enhance the immunogenicity of molecules
without altering their chemical composition.

12. Uses of adjuvants in immunology
Adjuvants affect the immune response in various ways:
• To increase the immunogenicity of weak antigens
• To enhance speed and duration of immune response
• To stimulate and modulate humoral responses, including
antibody isotype
• To stimulate cell-mediated immunity
• Enhance immune responses in immunologically
immature patients, particularly infants
• To decrease the dose of antigen required; reducing costs
and eliminating inconvenient requirements for booster
shots

13. Summary of adjuvants approved for human use
Adjuvant Description
Approved vaccine
products
Aluminium-based
mineral salts (Alum)
Aluminium phosphate,
Calcium phosphate,
Aluminium hydroxide
-Anthrax
-Hepatitis A
MF59
Submicron oil-in-water
emulsion
Influenza
Monophosphoryl lipid A
(MPL)
Bacteria-derived
immunostimulant
Hepatitis
Virosomes
Spherical vesicles
containing viral
membrane proteins in
the lipid membrane
Hepatitis
Influenza

14. Classification of antigens
Antigens can be classified based on many characters:
i. Based on their origin
ii. Based on chemical nature and structure
iii. Based on their Immunogenicity
iv. Based on cellular response generated
I. Based on their origin
 Exogenous antigens
 Endogenous antigens
 Autoantigens

15. Exogenous antigens:
• Enters the body from outside  inhalation, ingestion or injection
• Enter cells by endocytosis or phagocytosis and starts circulating in
the body fluids and trapped by the APC
• Some antigens starts as exogenous endogenous e.g intracellular
viruses, bacteria, and fungi.
Endogenous antigens:
• Body’s own cells or antigenic products that are produced from
normal cell metabolism
• Processed by macrophages which are later accepted by cytotoxic
T-cells. Includes xenogeneic, autologous and isoantigenic/allogenic
antigens e.g Blood group antigens, HLA(Histocompactibility
leukocyte antigen

16. Autoantigens
• Antigens that stimulates autoantibodies in the organism
that produce it
• Are self antigens that involved in autoimmune disease
pathogenesis
• Usually normal proteins/complex of proteins that is
recognized by the immune system as nonself, E.g
Nucleoproteins, NA.
• Another good example is p53 which have been described
as an autoantigen in several autoimmune diseases eg SLE
and many cancer diseases

17. II. Based on chemical nature and structure
• Proteins
– Majority of immunogens are proteins
– Proteins are usually very good immunogens.
• Polysaccharides
– Pure polysaccharides and lipopolysaccharides are good
immunogens.
• Nucleic Acids/Nucleoproteins
– Nucleic acids are usually poorly immunogenic..
• Lipids
– In general lipids are non-immunogenic but may be haptens.

18. III. Immunogenicity(Immune response)
Complete antigen (Immunogen):
 Able to generate immune response by themselves
High molecular weight >10000D
Proteins or polysaccharides
Incomplete antigen (hapten):
 Can not induce immune response on their own,
needs a carrier molecule
Low molecular weight

19. IV. Based on cellular response generated
Biological classes of antigens
• Depending on the mechanisms of inducing antibody formation,
antigens are classified as:
 T cell independent (TI) antigens.
 T cell dependent (TD) antigens.
 Superantigens
T-independent antigens:
• Can directly stimulate B cells to produce Ab without the help of
T-cells
• Generally they are polysaccharides
• Induce B cells to produce only Ig M
• No immune memory
• Can not induce CMI

20. Properties of T-independent antigens
Polymeric structure:
• Same antigenic determinant epitope repeated many times
Polyclonal activation of B cells:
• Can activate B cell clones specific for other Ags
• Subdivided into type 1 and 2 based on their ability to
polyclonally activate B cells
• Type 1 are polyclonal activators while type 2 are not.
Resistance to degradation:
• More resistant to degradation persists for a longer
period of time and continue to stimulate the immune
system.

21. T-cell dependent antigens
• Most of the normal antigens are T cell dependent, they
are processed and presented by antigen-presenting cells
(APCs) to T cells which leads to T cell activation.
• Activated T cells secrete cytokines that in turn stimulate
the B cells to produce antibodies.
• Do not directly stimulate the production of Ab without
the help of T cells.
• Generally they are proteins
• Stimulate B cells to produce IgM, IgG and Ig A
• Can induce CMI, have immune memory

23. Superantigens
• Superantigens are the third class of biological antigens.
• There are different types such as bacterial, viral and
fungal superantigens
• The unique feature of superantigens is, they can
activate T cells directly without being processed by
antigen presenting cells (APCs).
• Intact protein binds to variable region of β chain on TCR
of T cells and to MHC class II on antigen presenting cells
(APC)

26. Immunogens
• To protect against infectious disease, the immune system recognize
bacteria, bacterial products, fungi, parasites, and viruses as
immunogens.
• Proteins are the most potent immunogens, with polysaccharides
ranking second.
• Humoral immunity experimental studies use proteins or
polysaccharides as immunogens.
• For cell-mediated immunity, only proteins and some lipids and
glycolipids serve as immunogens.
• Proteins must first be processed into small peptides and then
presented together with MHC molecules on the membrane of a cell
before they can be recognized as immunogens.

27. Factors Influencing Immunogenicity
Immunogenicity is not an intrinsic property of an
antigen but rather depends on a number of properties
of the particular biological system that the antigen
encounters
• The factors includes:
i. The Nature of the Immunogen
ii. The Biological System
iii. Mode of administration

28. I. Nature of the Immunogen
Foreignness:
• In order to elicit an immune response, a molecule must be
recognized as non-self by the biological system.
• The capacity to recognize nonself is accompanied by
tolerance of self, a specific unresponsiveness to self
antigens.
Molecular size:
• The most potent immunogens are usually large proteins, have a
molecular mass > 10,000Da
• Generally, substances with a molecular mass less than 5000 Da are
poor immunogens.
• And very small ones (e.g, amino acids, Haptens) are non immunogenic

29. Chemical nature and composition:
• In general the more complex the substance is chemically the more
immunogenic it will be
• Immunogenicity Order;
Proteins > carbohydrates > lipid > nucleic acids.
Structural complexity:
• Simple homopolymers made up of single amino acid lack
immunogenicity.
• Polymers made up of two or more amino acids are immunogenic.
• Addition of aromatic amino acids increases immunogenicity.
Antigen specificity:
• Antigen specificity depends on the specific active sites on the
antigenic molecule

30. Degradability:
• Antigens that are easily phagocytosed are generally more
immunogenic
• Macromolecules that cannot be degraded and presented
with MHC molecules are poor immunogens.
Auto specificity:
• Autologous or self antigens are ordinarily not
immunogenic but under certain circumstances may act as
auto antigens

31. II. The biological system
Species Specificity:
• Tissues of all individuals in a particular species
possess species specific antigens
• Human blood group proteins can be differentiated
from animal protein by specific antigen-antibody
reactions
Organ specificity:
• Organ specific Antigens are confined to a
particular organ/tissue

32. Genetic factors:
• The genetic constitution (genotype) of host influences the type and
degree of immune response.
• Different individuals of a given species show different types of
immune responses towards the same antigen.
• Responders- are the individuals who produce antibody faster
• Slow responders- are the individuals who produce antibody
slowly and may need repeated antigenic exposures
• Non-responders - are the individuals who do not produce
antibody in spite of repeated antigenic exposures.
• MHC gene products which present processed antigen to T cells
play a central role in determining the degree of host immune
response to immunogen

33. Age:
• Usually the very young and the very old
individuals have a diminished ability to elicit an
immune response due to weak immune system as
in children immune system is still in
development and in old there is reduced
production of B and T cells in the bone marrow
and thymus and diminished function of mature
lymphocytes in secondary lymphoid tissues.

34. III. Mode of administration
Dose of the antigen:
• Optimal dose is required to stimulate host immune
response
• Achieved by repeated administration or boosters over a
period of weeks
• A single dose of immunogen (under dose) may not induce
a strong response.
Route of administration:
• Parenteral route & Intraperitoneal are preferred over oral.
• Combination of optimal dosage and route of administration will
induce a peak immune response in a given host.

35. Adjuvants:
• Adjuvants are substances that stimulate the immune response by
facilitating uptake into APCs
• 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.

36. Haptens
• Haptens are small organic
molecules that are antigenic
but not immunogenic by
itself.
• Chemical coupling of a
hapten to a large protein,
called a carrier, yields an
immunogenic hapten-carrier
conjugate which produce
antibodies specific for
(immunogens)
• A hapten-carrier conjugate
contains multiple copies of
the hapten

37. Why knowing Haptens important
• Many biologically important substances, including drugs, peptide
hormones, and steroid hormones, can function as haptens.
• Conjugates of these haptens with large protein carriers produce
hapten-specific antibodies useful for measuring the presence of
various substances in the body
• Formation of drug-protein conjugates in the body can produce
drug allergies that may be life-threatening.

39. References
• TEXTBOOK: JONATHAN M. AUSTYN AND KATHRYN J. WOOD
Principles of Cellular and Molecular Immunology .Chapter 2. pg 63-84.
• TEXTBOOK: ABUL K. ABBAS. ANDREW H. LICHTMAN. CELLULAR AND
MOLECULAR IMMUNOLOGY. 5TH EDITION. pg 58, 216, 489.
• Prescott Harley Microbiology 5th Edition
• Medical Microbiology Patrick Murray 9th Edition,2023
• Immunology by Tapasya Srivastava and Subrata Sinha
• Online Microbiology notes, Antigen Properties, Types and
determinants of Antigenicity
• Immunobiology jenways 10th ed,2023