Antigen: Structure, Types and Factors Affecting Immunogenicity 5/5 (2)

The substance that induce a specific immune response and subsequently react with the products of a specific immune response is called antigen (it is more appropriately called as immunogen).

In the case of infectious diseases, the antigens are components of invading microorganism’s structure that are usually composed of proteins or polysaccharides.

Ag has to be recognized by the

  • Immunoglobulin receptor of B Cells or
  • By the T cell receptor when complexed with MHC

Among the biological macromolecules, protein is the most potent immunogen followed by the polysaccharide. Other macromolecules such as lipids and nucleic acids do not serve as immunogen.


For Cell Mediated immunity only proteins and some lipids/glycolipids serve as immunogen.

Properties of an Immunogen

  • Immunogenicity
  • Antigenicity

Immunogenicity: Ability to induce a humoral and/or cell mediated immune response.

  • B cells + Ag   =           effector B cells (plasma cells) + memory B cells
  • T cells + Ag    =           effector T cells (e.g., CTLs, THs) + memory T cells

Antigenicity: The ability to combine/react specifically with the final products of the above responses (i.e., antibodies and/or cell-surface receptors).

All molecules that have the property of immunogenicity also have the property of antigenicity but Reverse not true. Remember: All Immunogen are Antigen but all Antigen are not Immunogen e.g. Hapten.

Hapten are antigenic but incapable by themselves of inducing a specific immune response, i.e., they lack immunogenicity


  • Immune cells do not interact with or recognize an entire immunogen instead they recognize discrete sites on the macromolecule which is called epitopes. 
  • Epitopes are immunologically active regions of an immunogen that bind to antigen-specific membrane receptors on lymphocytes or to secreted antibodies.
  • B cells and T cells recognize different epitopes on the same antigenic molecule.
  • B cell bind epitopes that are present in highly accessible sites on the exposed surface of the immunogen.
  • T cells recognize only peptides combined with MHC molecules on the surface of APCs or altered self cells.


  • A substance that is non-immunogenic but which can react with the products of a specific immune response.
  • Haptens are small molecules which could never induce an immune response when administered by themselves but which can when coupled to a carrier molecule.
  • Haptens have the property of antigenicity but not immunogenicity.


The substances that are least immunogenic are

a) Proteins

b) Polysaccharides

c) Nucleic acids

d) None of the above

Factors influencing Immunogenicity

Nature of the Immunogen

  1. Foreignness
  2. Molecular size
  3. Chemical composition and heterogeneity
  4. Ability to be  processed and presented with an MHC molecule on the surface of APC or altered self-cell

Biological system that the antigen encounters

  1. Genotype of the recipient animal
  2. Dosage and route of administration

Factors influencing Immunogenicity

Ⅰ. Foreignness

Must be recognized as non-self by the biological system

Degree of immunogenicity depends on the degree of foreignness    i.e. The greater the phylogenetic distances between two species, the greater the structural (and therefore the antigenic) disparity between them.

e.g. If Bovine serum albumin is injected in Cow, Rabbit and Chicken, the order of Immunogenicity will be:

        Cow < Rabbit < Chicken (least for cow and most for chicken)

This property is govern by: Tolerance to self (specific unresponsiveness to self antigens)

Concept of tolerance

During lymphocyte development immature lymphocyte are exposed to self-components.

  1. Those that reacts with self antigen are killed (clonal deletion)
  2. Antigens that have not been exposed to immature lymphocytes during this critical period may be later recognized as nonself, or foreign, by the immune system.

Exceptions: Conserved macromolecules across species (e.g. Collagen, Cytochrome C:  no immunogenicity)

Remember: Sequestered Antigen are treated as foreign (eg. Corneal tissue, sperms) e.g. aspermatogenesis


Ⅱ.  Molecular Size

Correlation  exists between size of the macromolecule and its immunogenicity

1.Molecular Mass  ≥ 1,00,000 Da:  Active Immunogens
2. Molecular Mass 5000-10,000 Da: Poor immunogen

Exceptions: Few substances with molecular mass less than 1000 Da have proven to be immunogenic.

III. Chemical Composition and Heterogeneity

  • Chemical complexity contributes to immunogenicity
  • Copolymers composed of different amino acids or sugars are usually more immunogenic than homopolymer of their constituents.
  • All four levels of protein organization contribute to the structural complexity of a protein and hence affect its immunogenicity.

Susceptibility to Ag Processing and Presentation

  • The development of both humoral and cell-mediated immune response requires interaction of T cells with Ag that has been processed and presented together with MHC molecules.
  • Large, insoluble macromolecules > Small, soluble macromolecules
    • Ease of phagocytosis and processing
  • Can not degraded & presented: Poor immunogen e.g. Polymers of D-amino acids

Contribution of the Biological System

1.Age: Usually the very young and the very old have a diminished ability to mount an immune response in response to an immunogen.

2.Genotype of the recipient animal:

  • Genetic constitution of an immunized animal influences the type of immune response the animal manifests, as well as the degree of response.
  • Genetic control of immune responsiveness is largely confined to genes within MHC.
  • MHC gene products plays central role in determining the degree to which an animal responds to an immunogen.
  • Some substances are immunogenic in one individual but not in others (i.e.responders and non-responders).

Immunogen Dosage and Route of Administration

A: Amount of Immunogen 

  • Optimum dose is necessary to mount good immune response.
  • Insufficient dose: will not stimulate an immune response (fails to activate enough lymphocyte or can induce a state of immunologic unresponsiveness, or tolerance).
  • Excessively high dose: Induces tolerance

B: Times

  • Single dose: will not induce strong response
  • Repeated administration (boosters): Increases clonal proliferation of antigen specific T cells or B cells and thus increase the lymphocyte populations specific for the immunogen.
Primary Vs. Secondary Immune Response

C. Routes of Administration:

  • Generally the subcutaneous route is better than the intravenous or intragastric routes
  • Administration route strongly influences which immune organs and cell populations will be involved in the response.

e.g. intravenous  (spleen); subcutaneous (local lymph nodes)

Use of Antigen with or without Adjuvants


  • Substances that when mixed with an antigen and injected with it, enhance the immunogenicity of that antigen.
  • Used to boost the immune response when an Ag has low immunogenicity or when small amount of Ag are available
  • Adjuvants appear to exert one or more of the following effects;
    • Antigen persistence is prolonged  (e.g. alum, water in oil adjuvants)
    • Co-stimulatory signals are enhanced
    • Local inflammation is increased
    • Nonspecific proliferation of lymphocytes is stimulated

Freund’s Adjuvant

  • one of the most commonly used adjuvants
  • designed to provide continuous release of antigens necessary for stimulating a strong, persistent immune response.
  • It is used as a water-in-oil emulsion.
  • It is prepared from non-metabolizable oils (paraffin oil and mannide monooleate).

Main disadvantage: it can cause granulomas, inflammation at the inoculation site and lesions.


Freund’s incomplete adjuvant

  • Ag in aqueous solution, mineral oil, mannide monooleate (emulsifying agent)
  • Small droplets of oil surrounding Ag formed
  • Ag is released slowly from the site of injection

Freund’s complete adjuvant

  • Highly effective adjuvant
  • Incomplete adjuvant+ heat-killed Mycobacterium tuberculosis 
  • Muramyl dipeptide ( a component of mycobacterial cell wall), activates macrophages, making Freund’s complete adjuvant far more potent than incomplete form.

Types of antigens

T Dependent Antigen (Td-Ag)

  • Antigens that require co-stimulation by a helper T-cell to activate a B-cell
  • Usually proteins
  • The activated B-cell clonally proliferates to produce a population of plasma cells and memory cells, which all recognize the same antigen

T Independent Antigen (Ti-Ag)

  • Antigens that stimulate B-cells directly, without co-stimulation by helper T-cells
  • Usually polysaccharides or lipopolysaccharides (e.g. bacterial capsules)
  • Cross link antigen receptors on the surface of B-cells to activate them
  • Don’t generate strong immune response (no memory cells, IgM is the only antibody class produced, and the immunity doesn’t last long).

Super Antigen (SAg)

  • Polyclonally activate a large fraction (up to 25%) of the T cells, setting off massive immune response
  • In T dependent Antigen only a fraction of (1 in 106 -104) of the T cell population is able to recognize the antigen and become activated

Examples of superantigens include:

  • Staphylococcal enterotoxins
  • Staphylococcal toxic shock toxin (TSST-1)
  • Streptococcal pyrogenic exotoxins (exotoxin A and exotoxin B)


Heterophilic Antigen

1.An antigen that is possessed by a variety of different phylogenetically unrelated species. Antibodies induced by these antigens cross-react with individual heterophilic antigens.
This antigen type is involved in the pathogenesis of certain diseases, e.g. infectious mononucleosis, rheumatic fever, glomerulonephritis.

e.g. Antigen of Group A beta hemolytic streptococci (Streptococcus pyogenes) and antigens of the human myocardium are heterophilic

Examples of Heterophile antigens

  • Forssman antigen: Lipid carbohydrate complex widely distributed in man, animals, birds, plants and bacteria.
  • Weil-Felix reaction: Used in the diagnosis of Typhus fever.  Antibodies produced against Rickettsia is detected by using Proteus antigen.
  • Paul-Bunnell test (Monospot test): a rapid test for the diagnosis of infectious mononucleosis. Hetrophile antibodies agglutinate sheep erythrocytes.
  • Cold agglutinin test: Agglutination of human O group erythrocytes at 4oC by the sera of patients suffering from primary atypical pneumonia caused by Mycoplasma pneumoniae.

Antigen in disease Diagnosis

  • We can detect the antigen (bacterial, fungal or viral) from the clinical sample.E.g. HBsAg, HBeAg, PLDH, HRP-2, Rapid Strep test, Crytococcal Antigen testing
  • We can use specific antigen and detect the antibody produced by our body. rK-39  test, Widal Test, TPHA, ASO titre etc.


Paul-Bunnell test is based on sharing of antigens between:

  • a.Sheep erythrocytes and Epstein Barr Virus
  • b.Mycoplasma and Human RBC
  • c.Ricekttsial antigen and Proteus antigen
  • d.None of the above

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