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11/04/2024 AIIMS Rajkot 1
ANTIGEN
Dr. Mayuri Bhise
Department of Microbiology
AIIMS Rajkot
11/04/2024 AIIMS Rajkot 2
Antigen
Defined as any substance that satisfies two distinct immunologic properties-
Immunogenicity
Antigenicity.
11/04/2024 AIIMS Rajkot 3
Immunogenicity:
Ability of an antigen to induce immune response in the 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
Substance that satisfies immunogenicity - more appropriately called as ‘immunogen’ rather
than ‘antigen’.
Introduction –Cont…
11/04/2024 AIIMS Rajkot 4
Antigenicity (immunological reactivity):
Ability of an antigen to combine specifically with the final products antibodies and/or T cell-surface
receptors.
All molecules having immunogenicity property, also show antigenicity, but the reverse is not true
E.g. Haptens- which are antigenic but not immunogenic.
11/04/2024 AIIMS Rajkot 5
Epitope
Smallest unit of antigenicity.
Definition - Small area present on the antigen comprising of few (four to five) amino acids or
monosaccharide residues, that is capable of sensitizing T and B cells and reacting with
specific site of T cell receptor or an antibody.
Specific site of an antibody that reacts with the corresponding epitope of an antigen is called
as paratope.
Types of epitope:
Sequential or linear epitope- Present as a single linear sequence of few amino acid residues.
11/04/2024 AIIMS Rajkot 6
Types of epitope:
Sequential or linear epitope- Present as a single linear sequence of few amino acid residues.
Conformational or non sequential epitopes:
Found on the flexible region of complex antigens having tertiary structures.
Formed by bringing together the surface residues from different sites of the peptide chain
during its folding into tertiary structure.
11/04/2024 AIIMS Rajkot 7
Hapten
Low molecular weight molecules that lack immunogenicity (cannot induce immune response) but
retain antigenicity or immunological reactivity (i.e. can bind to their specific antibody or T cell
receptor).
Haptens can become immunogenic when combined with a larger protein molecule called carrier.
Classification:
Complex haptens - contain two or more epitopes.
Simple haptens - contain only one epitope
11/04/2024 AIIMS Rajkot 8
Antigen Host Relationship
Based on the antigen-host relationship, antigens can be grouped into two groups:
Self or auto antigens
Non-self or foreign antigens
11/04/2024 AIIMS Rajkot 9
Self or auto antigens:
Belong to the host itself - not immunogenic.
Hosts do not react to their own antigens by exhibiting a mechanism called immunological
tolerance.
Sometimes, the self-antigens are biologically altered (e.g. as in cancer cells) and can become
immunogenic.
11/04/2024 AIIMS Rajkot 10
Non-self or foreign antigens:
Immunogenic and are of three types based on their phylogenetic distance to the host.
Alloantigens
Isoantigens ---Blood group — Human erythrocyte antigen
Histocompatibility antigen — Transplantation
Heteroantigens
11/04/2024 AIIMS Rajkot 11
Heterophile Antigens
Type of Heteroantigens that are present in two different species; but they
share epitopes with each other.
Antibody produced against antigen of one species can react with the
other and vice versa.
11/04/2024 AIIMS Rajkot 12
Diagnostic application
Heterophile antigens can be used in various serological tests.
Antibody against one antigen can be detected in patient’s serum by employing a different antigen
which is heterophile (cross reactive) to the first antigen. Examples:
Weil-Felix reaction
Paul-Bunnell test
Cold agglutination test and Streptococcus MG test
11/04/2024 AIIMS Rajkot 13
• Size of the antigen
• Chemical nature of the antigen
• Susceptibility of antigen to tissue enzymes
• Structural complexity
• Foreignness to the host
• Genetic factor
Factors affecting immunogenicity
11/04/2024 AIIMS Rajkot 14
CONT…
• Optimal dose of antigen
• Route of antigen administration:
• Repeated Number of doses of antigens
• Multiple antigens
• Effect of prior administration of antibody
11/04/2024 AIIMS Rajkot 15
Treatment
Size of the antigen:
Larger is the size (>10,000 Dalton molecular weight, e.g. hemoglobin); more potent is the molecule
as an immunogen
Chemical nature of the antigen:
Proteins are stronger immunogens than carbohydrates followed by lipids and nucleic acids
11/04/2024 AIIMS Rajkot 16
Cont…
Susceptibility of antigen to tissue enzymes :
• Only substances that are susceptible to the action of tissue enzymes are immunogenic.
• Degradation of the antigen by the tissue enzymes produces several immunogenic fragments
having more number of epitopes exposed.
Structural complexity:
• Simple homopolymers made up of single amino acid lack immunogenicity.
• Polymers made up of two or more amino acids are immunogenic.
11/04/2024 AIIMS Rajkot 17
Cont…
Foreignness to the host :
• Key factor which determines immunogenicity.
• Higher is the phylogenetic distance between the antigen and the host; more is the
immunogenicity.
Genetic factor:
• Different individuals of a given species show different types of immune responses towards
the same antigen.
Responders
Slow responders
Non-responders
11/04/2024 AIIMS Rajkot 18
Cont…
Optimal dose of antigen:
• An antigen is immunologically active only in the optimal dose range
Route of antigen administration:
• Immune response is better induced following parenteral administration of an
antigen.
• Depends on the type of antibody produced.
11/04/2024 AIIMS Rajkot 19
Cont…
Repeated doses of antigens:
• Repeated doses of antigens over a period of time are needed to generate an
adequate immune response.
• This is due to the role of memory cells in secondary immune response.
11/04/2024 AIIMS Rajkot 20
Multiple antigens:
When two or more antigens are administered simultaneously, the effects may vary.
Antibody response to one or the other antigen may be equal or diminished (due to
antigenic competition) or enhanced (due to adjuvant like action).
Effect of prior administration of antibody:
Primary immune response is more susceptible to get suppressed than the secondary
immune response.
Therapeutic application
Cont…
11/04/2024 AIIMS Rajkot 21
Any substance that enhances the immunogenicity of an antigen.
Added to vaccines to increase the immunogenicity of the vaccine antigen
Examples:
Alum (aluminium hydroxide or phosphate)
Mineral oil (liquid paraffin)
Freund's incomplete adjuvant- It is a water-in-oil emulsion containing a protein
antigen in the aqueous phase.
Freund's complete adjuvant is the mixture of Freund's incomplete adjuvant &
suspension of killed tubercle bacilli in the oil phase.
Adjuvant
11/04/2024 AIIMS Rajkot 22
Cont…
• Examples:
Lipopolysaccharide (LPS) fraction of Gram-negative bacilli
Other bacteria or their products-
Mycobacterium bovis
Toxoid (diphtheria toxoid and tetanus toxoid act as adjuvant for Haemophilus influenzae-
b vaccine)
Nonbacterial products: Silica particles, beryllium sulfate, squalene, and thimerosal.
Mechanism of adjuvant action:
1. Delaying the release of antigen
2. By activating phagocytosis
3. By activating TH cells
4. By granuloma formation
11/04/2024 AIIMS Rajkot 23
Biological classes of Antigen
• Depending on the mechanisms of inducing antibody formation, antigens are classified
as:
 T cell dependent (TD) antigens.
• Most of the normal antigens are T cell dependent, they are processed and presented by
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.
11/04/2024 AIIMS Rajkot 24
T cell independent (TI) antigens.
• Antigens such as bacterial capsule, flagella and LPS (lipopolysaccharide) that do
not need the help of T cells and APCs.
• Directly bind to immunoglobulin receptors present on B cells and stimulate B
cells polyclonally.
• Leads to increased secretion of non- specific antibodies (hypergamma-
globulinemia).
11/04/2024 AIIMS Rajkot 25
Differences
11/04/2024 AIIMS Rajkot 26
Cont…
11/04/2024 AIIMS Rajkot 27
Superantigens
• Superantigens are the third variety of biological class of antigens, recently
described in the last decade.
• Unique feature of superantigens is, they can activate T cells directly without
being processed by antigen presenting cells (APCs).
• The variable β region of T cell receptor (vβ of TCR) appears to be the receptor for
superantigens.
11/04/2024 AIIMS Rajkot 28
Cont…
• Directly bridge non-specifically between major histocompatibility complex
(MHC)-II of APCs and T cells.
• Non-specific activation of T cells leads to massive release of cytokines which can
activate B cell polyclonally, which leads to increased secretion of non- specific
antibodies (hypergammaglobulinemia)
11/04/2024 AIIMS Rajkot 29
Cont…
Do not require Ag’ processing by MØ /APC & presentation in the groove of MHC molecule.
Attach to MHC molecule on MØ from outer side & also to outer side of TCR simultaneously.
11/04/2024 AIIMS Rajkot 30
Cont…
11/04/2024 AIIMS Rajkot 31
11/04/2024 AIIMS Rajkot 32
Q
1
• Q1. Superantigen causes:
a. Enhancement of phagocytosis
b. Polyclonal activation of B cells
c. Antigen presentation by macrophage
d. Activation of complement
11/04/2024 AIIMS Rajkot 33
• Q2. Which part of the bacteria is most antigenic:
a. Protein
b. Carbohydrate
c. Lipid
d. Nucleic acid
11/04/2024 AIIMS Rajkot 34
• Q3. Which of the following antigen is T-dependent:
a. Bcaterial capsule
b. Flagella
c. Lipopolysaccharide
d. Exotoxin
11/04/2024 AIIMS Rajkot 35
Epidemiology
• Q4. Diagnostic application of heterophile antigens include all, except:
a. Weil-Felix reaction
b. Paul-Bunnell test
c. Widal test
d. Cold agglutination test

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ANTIGEN- SECTION IMMUNOLOGY DEPARTMENT OF MICROBIOLOGY

  • 1. 11/04/2024 AIIMS Rajkot 1 ANTIGEN Dr. Mayuri Bhise Department of Microbiology AIIMS Rajkot
  • 2. 11/04/2024 AIIMS Rajkot 2 Antigen Defined as any substance that satisfies two distinct immunologic properties- Immunogenicity Antigenicity.
  • 3. 11/04/2024 AIIMS Rajkot 3 Immunogenicity: Ability of an antigen to induce immune response in the 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 Substance that satisfies immunogenicity - more appropriately called as ‘immunogen’ rather than ‘antigen’. Introduction –Cont…
  • 4. 11/04/2024 AIIMS Rajkot 4 Antigenicity (immunological reactivity): Ability of an antigen to combine specifically with the final products antibodies and/or T cell-surface receptors. All molecules having immunogenicity property, also show antigenicity, but the reverse is not true E.g. Haptens- which are antigenic but not immunogenic.
  • 5. 11/04/2024 AIIMS Rajkot 5 Epitope Smallest unit of antigenicity. Definition - Small area present on the antigen comprising of few (four to five) amino acids or monosaccharide residues, that is capable of sensitizing T and B cells and reacting with specific site of T cell receptor or an antibody. Specific site of an antibody that reacts with the corresponding epitope of an antigen is called as paratope. Types of epitope: Sequential or linear epitope- Present as a single linear sequence of few amino acid residues.
  • 6. 11/04/2024 AIIMS Rajkot 6 Types of epitope: Sequential or linear epitope- Present as a single linear sequence of few amino acid residues. Conformational or non sequential epitopes: Found on the flexible region of complex antigens having tertiary structures. Formed by bringing together the surface residues from different sites of the peptide chain during its folding into tertiary structure.
  • 7. 11/04/2024 AIIMS Rajkot 7 Hapten Low molecular weight molecules that lack immunogenicity (cannot induce immune response) but retain antigenicity or immunological reactivity (i.e. can bind to their specific antibody or T cell receptor). Haptens can become immunogenic when combined with a larger protein molecule called carrier. Classification: Complex haptens - contain two or more epitopes. Simple haptens - contain only one epitope
  • 8. 11/04/2024 AIIMS Rajkot 8 Antigen Host Relationship Based on the antigen-host relationship, antigens can be grouped into two groups: Self or auto antigens Non-self or foreign antigens
  • 9. 11/04/2024 AIIMS Rajkot 9 Self or auto antigens: Belong to the host itself - not immunogenic. Hosts do not react to their own antigens by exhibiting a mechanism called immunological tolerance. Sometimes, the self-antigens are biologically altered (e.g. as in cancer cells) and can become immunogenic.
  • 10. 11/04/2024 AIIMS Rajkot 10 Non-self or foreign antigens: Immunogenic and are of three types based on their phylogenetic distance to the host. Alloantigens Isoantigens ---Blood group — Human erythrocyte antigen Histocompatibility antigen — Transplantation Heteroantigens
  • 11. 11/04/2024 AIIMS Rajkot 11 Heterophile Antigens Type of Heteroantigens that are present in two different species; but they share epitopes with each other. Antibody produced against antigen of one species can react with the other and vice versa.
  • 12. 11/04/2024 AIIMS Rajkot 12 Diagnostic application Heterophile antigens can be used in various serological tests. Antibody against one antigen can be detected in patient’s serum by employing a different antigen which is heterophile (cross reactive) to the first antigen. Examples: Weil-Felix reaction Paul-Bunnell test Cold agglutination test and Streptococcus MG test
  • 13. 11/04/2024 AIIMS Rajkot 13 • Size of the antigen • Chemical nature of the antigen • Susceptibility of antigen to tissue enzymes • Structural complexity • Foreignness to the host • Genetic factor Factors affecting immunogenicity
  • 14. 11/04/2024 AIIMS Rajkot 14 CONT… • Optimal dose of antigen • Route of antigen administration: • Repeated Number of doses of antigens • Multiple antigens • Effect of prior administration of antibody
  • 15. 11/04/2024 AIIMS Rajkot 15 Treatment Size of the antigen: Larger is the size (>10,000 Dalton molecular weight, e.g. hemoglobin); more potent is the molecule as an immunogen Chemical nature of the antigen: Proteins are stronger immunogens than carbohydrates followed by lipids and nucleic acids
  • 16. 11/04/2024 AIIMS Rajkot 16 Cont… Susceptibility of antigen to tissue enzymes : • Only substances that are susceptible to the action of tissue enzymes are immunogenic. • Degradation of the antigen by the tissue enzymes produces several immunogenic fragments having more number of epitopes exposed. Structural complexity: • Simple homopolymers made up of single amino acid lack immunogenicity. • Polymers made up of two or more amino acids are immunogenic.
  • 17. 11/04/2024 AIIMS Rajkot 17 Cont… Foreignness to the host : • Key factor which determines immunogenicity. • Higher is the phylogenetic distance between the antigen and the host; more is the immunogenicity. Genetic factor: • Different individuals of a given species show different types of immune responses towards the same antigen. Responders Slow responders Non-responders
  • 18. 11/04/2024 AIIMS Rajkot 18 Cont… Optimal dose of antigen: • An antigen is immunologically active only in the optimal dose range Route of antigen administration: • Immune response is better induced following parenteral administration of an antigen. • Depends on the type of antibody produced.
  • 19. 11/04/2024 AIIMS Rajkot 19 Cont… Repeated doses of antigens: • Repeated doses of antigens over a period of time are needed to generate an adequate immune response. • This is due to the role of memory cells in secondary immune response.
  • 20. 11/04/2024 AIIMS Rajkot 20 Multiple antigens: When two or more antigens are administered simultaneously, the effects may vary. Antibody response to one or the other antigen may be equal or diminished (due to antigenic competition) or enhanced (due to adjuvant like action). Effect of prior administration of antibody: Primary immune response is more susceptible to get suppressed than the secondary immune response. Therapeutic application Cont…
  • 21. 11/04/2024 AIIMS Rajkot 21 Any substance that enhances the immunogenicity of an antigen. Added to vaccines to increase the immunogenicity of the vaccine antigen Examples: Alum (aluminium hydroxide or phosphate) Mineral oil (liquid paraffin) Freund's incomplete adjuvant- It is a water-in-oil emulsion containing a protein antigen in the aqueous phase. Freund's complete adjuvant is the mixture of Freund's incomplete adjuvant & suspension of killed tubercle bacilli in the oil phase. Adjuvant
  • 22. 11/04/2024 AIIMS Rajkot 22 Cont… • Examples: Lipopolysaccharide (LPS) fraction of Gram-negative bacilli Other bacteria or their products- Mycobacterium bovis Toxoid (diphtheria toxoid and tetanus toxoid act as adjuvant for Haemophilus influenzae- b vaccine) Nonbacterial products: Silica particles, beryllium sulfate, squalene, and thimerosal. Mechanism of adjuvant action: 1. Delaying the release of antigen 2. By activating phagocytosis 3. By activating TH cells 4. By granuloma formation
  • 23. 11/04/2024 AIIMS Rajkot 23 Biological classes of Antigen • Depending on the mechanisms of inducing antibody formation, antigens are classified as:  T cell dependent (TD) antigens. • Most of the normal antigens are T cell dependent, they are processed and presented by 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.
  • 24. 11/04/2024 AIIMS Rajkot 24 T cell independent (TI) antigens. • Antigens such as bacterial capsule, flagella and LPS (lipopolysaccharide) that do not need the help of T cells and APCs. • Directly bind to immunoglobulin receptors present on B cells and stimulate B cells polyclonally. • Leads to increased secretion of non- specific antibodies (hypergamma- globulinemia).
  • 25. 11/04/2024 AIIMS Rajkot 25 Differences
  • 27. 11/04/2024 AIIMS Rajkot 27 Superantigens • Superantigens are the third variety of biological class of antigens, recently described in the last decade. • Unique feature of superantigens is, they can activate T cells directly without being processed by antigen presenting cells (APCs). • The variable β region of T cell receptor (vβ of TCR) appears to be the receptor for superantigens.
  • 28. 11/04/2024 AIIMS Rajkot 28 Cont… • Directly bridge non-specifically between major histocompatibility complex (MHC)-II of APCs and T cells. • Non-specific activation of T cells leads to massive release of cytokines which can activate B cell polyclonally, which leads to increased secretion of non- specific antibodies (hypergammaglobulinemia)
  • 29. 11/04/2024 AIIMS Rajkot 29 Cont… Do not require Ag’ processing by MØ /APC & presentation in the groove of MHC molecule. Attach to MHC molecule on MØ from outer side & also to outer side of TCR simultaneously.
  • 32. 11/04/2024 AIIMS Rajkot 32 Q 1 • Q1. Superantigen causes: a. Enhancement of phagocytosis b. Polyclonal activation of B cells c. Antigen presentation by macrophage d. Activation of complement
  • 33. 11/04/2024 AIIMS Rajkot 33 • Q2. Which part of the bacteria is most antigenic: a. Protein b. Carbohydrate c. Lipid d. Nucleic acid
  • 34. 11/04/2024 AIIMS Rajkot 34 • Q3. Which of the following antigen is T-dependent: a. Bcaterial capsule b. Flagella c. Lipopolysaccharide d. Exotoxin
  • 35. 11/04/2024 AIIMS Rajkot 35 Epidemiology • Q4. Diagnostic application of heterophile antigens include all, except: a. Weil-Felix reaction b. Paul-Bunnell test c. Widal test d. Cold agglutination test

Editor's Notes

  1. So, our study is designed to determine the baseline Widal titre among healthy general population in and around Rajkot
  2. Commercially available antigens (Typhocheck Tulip Diagnostics Ltd) using standardized suspension of S. enterica serotype typhi ‘O’ and ‘H’ and S. enterica serotype paratyphi A ‘H’ and S. enterica serotype paratyphi B ‘H’ antigen will be used.
  3. Commercially available antigens (Typhocheck Tulip Diagnostics Ltd) using standardized suspension of S. enterica serotype typhi ‘O’ and ‘H’ and S. enterica serotype paratyphi A ‘H’ and S. enterica serotype paratyphi B ‘H’ antigen will be used.