AntigenAntigen

Initiator of the immune response

The function of immune The function of immune systemsystem

Protect us from infections disease.(immune defence) pathogens: Viruses, Bacteria, Fungi, Worms, Parasites

Removes abnormal cells including cancer cells （ immune surveillance ） Promote normal functioning of the body （ immune homeostasis ） Tissue cleanup, wound repair

But the immune system can also cause disease when it does function improperly. Allergies, Immunodeficiency Disease, Autoimmunity,

The player of the immune The player of the immune systemsystem

lymphoid organs:lymphoid organs: Thymus, Bone marrow——primaryThymus, Bone marrow——primary Lymph nodes, Spleen——secondaryLymph nodes, Spleen——secondary Immune cellsImmune cells neutrophil, monocyte, macrophage, dendritic cells, NK neutrophil, monocyte, macrophage, dendritic cells, NK cells, mast cellscells, mast cells T cells, B cellsT cells, B cells Immune molecules Immune molecules Antibodies, Cytokines, Complements, MHCAntibodies, Cytokines, Complements, MHC

Innate immunityInnate immunity Consists of the defenses against infection that are ready for Consists of the defenses against infection that are ready for

immediate activation prior to attack by a pathogen.immediate activation prior to attack by a pathogen. predeployed before an infection and poised to prevent or predeployed before an infection and poised to prevent or

eliminate it.eliminate it. allow a rapid response to infectionallow a rapid response to infection is the first line of defense against pathogensis the first line of defense against pathogens is not specific is not specific neutrophil, monocyte, macrophage, dendritic neutrophil, monocyte, macrophage, dendritic cells, NK cells, mast cellscells, NK cells, mast cells

Two categories of Immune respone

Develops in response to infection and adapts to recognize, eliminate and then remember the invading pathogens.T and B cells Become more powerful following repeated encounters with the same antigen. - highly antigenic specificity: distinguish subtle differences among antigens.- Immunologic memory: a second encounter with the same antigen induces a heightened state of immune reactivity.- self-nonself recognition: immune system normally responds only to foreign antigen.

Adaptive immunity

Humoral immune responseHumoral immune response

Cellular immune responseCellular immune response

AgAg

TT

BB

EffectorEffectorT cellsT cells

PlasmaPlasmacellscells

antiboantiboddyy

Adaptive immunity

The content of The content of antigenantigen chapter chapter

The concept and properties of antigenThe concept and properties of antigen The molecule features of antigens that The molecule features of antigens that

influence in immunogenicity of antigeninfluence in immunogenicity of antigen The concept and the types of epitopeThe concept and the types of epitope The types of the antigen The types of the antigen The important antigen in medicineThe important antigen in medicine

Antigen ： Substances (molecules) that can induce an an immune response and can react specifically with products of immune response (antibodies or effector T cells) in vivo or in vitro..

T

B

Effector T cell

plasma

Antibody

Ag

How does T/B cell recognize the antigen?

BCR and TCR

BCR

TCR

DC or MMH

C

Antigen

Antigens are also defined specifically as Antigens are also defined specifically as

molecules that interact with B cells receptor (BCR) or molecules that interact with B cells receptor (BCR) or

with the T cell receptor(TCR) when complexed with with the T cell receptor(TCR) when complexed with

MHC.MHC.

T/BT/B T/B

Ag1 Ag2 Ag3

Ab1Ab2 Ab3

Antigens possess two immunologic properties:

Immunogenicity:

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

Ability to bind specifically with the final products of the immune response. i.e. secreted antibodies and effector T cells.

Antigenicity

All Antigens that have the property of immunogenicity also have the All Antigens that have the property of immunogenicity also have the property of antigenicity. property of antigenicity.

Is the reverse true? Is the reverse true?

HaptenHaptenConcept

Small molecules that can react with specific antibody, but are incapable by themselves of inducing a specific immune response. Include: drugs, peptide hormones, steroid hormones

CarrierHapten Conjugate＋Large protein

Haptens are valuable research and diagnostic tools

HCG( human chorionic gonadotropin): a sign of pregnancy

Properties of the antigen contribute to Properties of the antigen contribute to immunogenicityimmunogenicity

ForeignnessForeignness

Molecular SizeMolecular Size

Chemical composition and complexityChemical composition and complexity

Ability to be processed and presented by APCsAbility to be processed and presented by APCs

1. Foreignness Antigen must be recognized as non-self by the biological system. Intensity of the immune response depends on the phylogenetic distance.

BSA injectCow -

Goat +

Rabbit ++

Chicken ++

Immunogenicity

(Bovine serume albumin)

Some molecules are too well conserved: collagen. Some self components can be served as immunogens

Exception:

100 kDa Size is optimal for antigens

2. Molecular size

MW 100,000≻MW 100,000≻ daltons (Da)daltons (Da) strongstrong immunogenicityimmunogenicity

MW 4000-10000 DaMW 4000-10000 Da weak immunogenicityweak immunogenicity MW 4000 Da ≺MW 4000 Da ≺ poor immumogenspoor immumogens

There is a correlation between the size of a molecule and its immunogenicity

3. Chemical Composition

Synthetic homopolymers tend to lack immunogenicity

regardless of their size

heteropolymers are usually more immunogenic than

homopolymers

Chemical complexity contributes to immunogenicity

Antibody Reactivity to Lipid Molecules

Antigen must be processed and presented by APCs before is recognized by T cells.

Large, insoluble and aggregated Ags are best since they can be phagocytized and processed

4. Susceptibility to antigen processed and presentation

What molecules generate the best immune response ?

Proteins > Carbohydrates > Lipids and Nucleic acids

Specificity of the antigen Specificity of the antigen

T/BT/B T/B

Ag1 Ag2 Ag3

Ab1Ab2 Ab3

Antigen can only bind with corresponding antibody or Antigen can only bind with corresponding antibody or receptors on B or T cells.receptors on B or T cells. Antigen→specific lymphocyte→responseAntigen→specific lymphocyte→response

BCR/TCR

Specificity of the antigen determine the specificity of adaptive immune respose Specificity of the antigen is determined by epitopes of the antigen

Antibodies or T cells and B

cells do not react with an entire

antigenic molecule, Instead

they react only with discrete

sites on the molecule, called

an epitope or antigenic

determinant.

How does the antigen bind with the antibody or T/B cell ?

Computer simulation of an interaction between antibody and influenza virus antigen

Epitope (Antigenic determinant)Epitope (Antigenic determinant)

Epitopes are the immunologically

active regions of an antigen that

bind to antigen-specific membrane

receptors on lymphocytes (TCR or

BCR) or to secreted antibodies.

Four Levels of Protein Organization

An epitope on a protein antigen may involve elements of the primary, secondary, tertiary ,even quaternary structure of the protein.

· · linear / sequential epitopes linear / sequential epitopes Contain several contiguous amino acids in Contain several contiguous amino acids in a polypeptide chaina polypeptide chain

· · conformational / non-sequential conformational / non-sequential epitopesepitopes The residues that constitute these The residues that constitute these epitopes are far apart in the primary amino epitopes are far apart in the primary amino acid sequence but close together in the acid sequence but close together in the tertiary structure of the antigen.tertiary structure of the antigen.

The types of epitopeThe types of epitope

B-cell epitope• Be reconizied by B cell, it binds with BCR directly• BCR and antibodies react with intact, soluble molecules. so B cell epitopes must be highly accessible sites on the exposed surface of the antigen. • B-cell epitopes are usually made up of hydrophilic amino acids on the surface of the molecule• both linear epitope and conformational epitope can acted as B-cell epitopes.

B cell epitope and T cell epitope

T cell epitope • be recognized by T cell, it binds with TCR in the form of MHC- polypeptide complex

• linear epitopes• distributing in any part of antigen molecule. T- cell epitopes are often hidden within the interior of a protein and consist of predominantly hydrophobic amino acids.

The types of antigenThe types of antigen

Immunogen, Hapten, allergen, tolerogenImmunogen, Hapten, allergen, tolerogen

Protein, polysaccharide, nucleic acid, lipid, etcProtein, polysaccharide, nucleic acid, lipid, etc

Thymus-dependent antigen(TD-Ag) and Thymus-Thymus-dependent antigen(TD-Ag) and Thymus-

independent antigen(TI-Ag)independent antigen(TI-Ag) Xenoantigen, alloantigen, autoantigen Xenoantigen, alloantigen, autoantigen

1. Thymus-dependent 1. Thymus-dependent antigen(TD-Ag) antigen(TD-Ag)

An antigen that requires T An antigen that requires T cell and B cell cell and B cell cooperation to induce cooperation to induce specific antibody specific antibody formation. formation.

TD-Ag Include: cells, blood cells, serum proteins, TD-Ag Include: cells, blood cells, serum proteins, bacteria and almost all the proteins. bacteria and almost all the proteins.

The structure of TD-Ag is more complex.The structure of TD-Ag is more complex.

Features: (1) require T cells activationFeatures: (1) require T cells activation

(2) induce cellular immunity and (2) induce cellular immunity and humoral immunity humoral immunity

(3) antibody types: IgM, IgG, etc(3) antibody types: IgM, IgG, etc

(4) apparent immune memory(4) apparent immune memory

2. Thymus-independent antigens(TI-Ag)2. Thymus-independent antigens(TI-Ag)

TI-Ag: An antigen that can be recognized by B cells, TI-Ag: An antigen that can be recognized by B cells, does not require T cells assisting in the does not require T cells assisting in the production of antibody. production of antibody. Include: Lipopolysaccharides (LPS), Include: Lipopolysaccharides (LPS), polymeric bacterial flagellin.polymeric bacterial flagellin.

FeaturesFeatures ： ： (1) not require T cell activation(1) not require T cell activation (2) only induce humoral immunity, not (2) only induce humoral immunity, not

cellular immunity cellular immunity (3) antibody type: IgM(3) antibody type: IgM (4) no apparent memory(4) no apparent memory

1. Xenoantigen1. Xenoantigen

(1) pathogenic microorganisms(1) pathogenic microorganisms::

Important Antigens in MedicineImportant Antigens in Medicine

— — 208 viruses208 viruses

— — 538 bacteria 538 bacteria

— — 57 parasite57 parasite

— — 317 fungi 317 fungi

— — 287 worms287 worms

exotoxin: is a toxin secreted by bacteria . An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism. Such as: tetanus-toxin 、 diphtheria toxintoxoid: is a bacterial toxin (usually an exotoxin) whose toxicity has been inactivated or suppressed either by chemical or heat treatment, while other properties, typically immunogenicity, are maintained. Such as: tetanus-toxoid 、 diphtheria-toxoid

(2) Exotoxin and toxoid

(3) animal immune serum /antiserum contain specific antibodies.

such as: tetanus-antitoxin, diphtheria-antitoxin

Horse serum

hypersensitivity

Antiserum

exotoxin Toxoid

Preparation of antiserum

2. Alloantigen

An antigen present in different member of the same species. (1)   ABO blood group (1)   ABO blood group antigens system.antigens system.(2)   Rh antigen(2)   Rh antigen(3)   HLA (human leukocyte (3)   HLA (human leukocyte antigen)antigen)

3. Heterophile antigen (Forssman antigen)3. Heterophile antigen (Forssman antigen)

Heterophile antigens are the shared antigens that Heterophile antigens are the shared antigens that present in many species, such as human, animal, plant present in many species, such as human, animal, plant as well as microorganisms, without species specificity.as well as microorganisms, without species specificity. streptococcus kidneystreptococcus kidney 、、 heartheart

Nephritis

4. 4. AutoantigensAutoantigensRA (Rheumatoid arthritis), Multiple sclerosis, Lupus RA (Rheumatoid arthritis), Multiple sclerosis, Lupus erythematosis.erythematosis.

(1) releasing of sequestered antigen(1) releasing of sequestered antigen such as: brain, protein of the lens of eye, sperm, such as: brain, protein of the lens of eye, sperm, thyroid globulin normally sequestered from thyroid globulin normally sequestered from the circulation the circulation

 (2) modified self tissue.  (2) modified self tissue. drugs drugs radiation self tissue autoantigens radiation self tissue autoantigens infection infection

5. Tumor antigens: antigens on tumor cells5. Tumor antigens: antigens on tumor cells (1) tumor specific antigen (TSA) (1) tumor specific antigen (TSA) are unique to tumor cells and do not occur on are unique to tumor cells and do not occur on normal cells in the body normal cells in the body (2) tumor associated antigen (TAA) (2) tumor associated antigen (TAA) are not unique to tumor cells, may be proteins that are are not unique to tumor cells, may be proteins that are expressed on normal cells during fetal development, not expressed on normal cells during fetal development, not expressed in the adult normally. But expressed increasely on expressed in the adult normally. But expressed increasely on tumor cells. embryon antigen tumor cells. embryon antigen such as AFP alpha-fetoprotein such as AFP alpha-fetoprotein CEA carcinoembryonic antigen CEA carcinoembryonic antigen (3) tumor antigens may be induced by virus: virus antigens (3) tumor antigens may be induced by virus: virus antigens such as EBV—Burkitt tumor such as EBV—Burkitt tumor nasopharynx cancer nasopharynx cancer HBV—liver cancer HBV—liver cancer HPV—cervical cancer HPV—cervical cancer

Non-specific stimulatorNon-specific stimulator

Superantigen are viral or bacterial proteins that bind simultaneously to the V domain of a T-cell receptor and to chain of class II MHC molecule.

Superantigen (SAg)

Type of SAgType of SAg Exogenous SAg: Exogenous SAg: soluble proteins soluble proteins secreted secreted by bacteria. by bacteria. (exotoxin)(exotoxin)

Endogenous SAg: Endogenous SAg: cell membrane protein encoded by certain viruscell membrane protein encoded by certain virus

Cross-linkage of a T-cell receptor and class II MHC Cross-linkage of a T-cell receptor and class II MHC molecule by either type of SAg produces an molecule by either type of SAg produces an activating signal that induces T –cell activation and activating signal that induces T –cell activation and prolieration.prolieration.

MMassive immune response that caused by SAgs is nassive immune response that caused by SAgs is not specific to any particular epitope on the SAg.ot specific to any particular epitope on the SAg.

Normal antigen: 0.001-0.0001% of T-cell SAg: up to 20% T-cells.

Mitogens ( polyclonal T/B activator)

The most commonly used mitogens in clinical laboratory medicine

The chemical substance that encourages a cell to commence cell division, triggering mitosis.

are able to activate all members of T/B cells regardless of their antigenic specificity.

Mitogens are often used to stimulate lymphocytes and therefore assess immune function.

The end The end