Immunogens, Antigens, and Immunogens, Antigens, and HaptensHaptens

Initiation of immune responseInitiation of immune response Interaction between receptor and ligandInteraction between receptor and ligand

AffinityAffinity Avidity Avidity

strongbinding

strongbinding

weakbinding

Affinity: high low low

IntroductionIntroduction

Immune responses arise as a result of exposure to Immune responses arise as a result of exposure to foreign stimuliforeign stimuli

The compound that evokes an immune response is The compound that evokes an immune response is referred to as “antigen” or “immunogen.”referred to as “antigen” or “immunogen.”

The distinction between the two is functional but The distinction between the two is functional but they are commonly used as synonyms. they are commonly used as synonyms.

Definitions An immunogenAn immunogen is any substance capable of is any substance capable of

inducing an immune responseinducing an immune response

An antigenAn antigen is any substance capable of binding is any substance capable of binding specifically to the products of the immune responsespecifically to the products of the immune response

All immunogens are antigens but not all antigens All immunogens are antigens but not all antigens need be immunogensneed be immunogens

Special Types of AntigensSpecial Types of Antigens AllergenAllergen MitogenMitogen Super antigenSuper antigen TolerogenTolerogen According to source of antigen:According to source of antigen: - Xenoantigen- Xenoantigen - Heteroantigen- Heteroantigen - Alloantigen- Alloantigen - Autoantigen - Autoantigen

HaptensHaptens are low molecular weight compounds are low molecular weight compounds (antibiotics and drugs) that by themselves are (antibiotics and drugs) that by themselves are incapable of inducing an immune response, but they incapable of inducing an immune response, but they can react with its productscan react with its products

When haptens are coupled with large molecules When haptens are coupled with large molecules such as proteins (carriers), the resultant conjugate such as proteins (carriers), the resultant conjugate induces an immune response directed against the induces an immune response directed against the hapten and the carrierhapten and the carrier

Factors influencing immunogenicityFactors influencing immunogenicity

Contribution of the immunogenContribution of the immunogen

ForeignnessForeignness High Molecular WeightHigh Molecular Weight - <1000 Daltons : nonimmunogenic- <1000 Daltons : nonimmunogenic - 1000-6000 Daltons: may be immunogenic- 1000-6000 Daltons: may be immunogenic - > 6000 immunogenic- > 6000 immunogenic

Chemical Nature and ComplexityChemical Nature and Complexity -- Homopolymers Vs HeteropolymersHomopolymers Vs Heteropolymers - Primary, secondary, tertiary, and - Primary, secondary, tertiary, and quaternary structuresstructures

Antigenic Determinants or EpitopesAntigenic Determinants or Epitopes - Linear- Linear - Discontinuous- Discontinuous Paratope: “The site in the variable (V) domain of an Paratope: “The site in the variable (V) domain of an

antibody or T-cell receptor that binds to an epitope antibody or T-cell receptor that binds to an epitope on an antigenon an antigen

Physical FormPhysical Form Particulate > SolubleParticulate > Soluble Denatured > NativeDenatured > Native

DegradabilityDegradability Ag processing by Ag Presenting Cells (APC)Ag processing by Ag Presenting Cells (APC)

Factors Influencing ImmunogenicityFactors Influencing ImmunogenicityContribution of the Biological SystemContribution of the Biological System

GeneticsGenetics SpeciesSpecies IndividualIndividual

Responders vs. Non-respondersResponders vs. Non-responders

AgeAge

Factors Influencing ImmunogenicityFactors Influencing ImmunogenicityMethod of AdministrationMethod of Administration

DoseDose

RouteRoute Subcutaneous > Intravenous > IntragastricSubcutaneous > Intravenous > Intragastric

Rate of eliminationRate of elimination

AdjuvantAdjuvant Substances that enhance an immune response to an AgSubstances that enhance an immune response to an Ag

AdjuvantsAdjuvants Substances which when mixed with an immunogen enhance Substances which when mixed with an immunogen enhance

the immune response against the immunogenthe immune response against the immunogen

They differ from carriers as they do not enhance immunity They differ from carriers as they do not enhance immunity to haptensto haptens

Release immunogens slowly but continuouslyRelease immunogens slowly but continuously

Types: Freund’s incomplete or complete adjuvants, BCG, Types: Freund’s incomplete or complete adjuvants, BCG, Corynebacterium parvumCorynebacterium parvum, , Bordetella pertussisBordetella pertussis, LPS, and , LPS, and Alum precipitate (most widely used )Alum precipitate (most widely used )

Major Classes of ImmunogensMajor Classes of Immunogens

ProteinsProteins: Best immunogens: Best immunogens

CarbohydratesCarbohydrates: Usually but not always good : Usually but not always good immunogensimmunogens

Nucleic AcidsNucleic Acids: Poor immunogens by themselves : Poor immunogens by themselves unless coupled to carriersunless coupled to carriers

LipidsLipids: Non immunogens unless coupled to carriers: Non immunogens unless coupled to carriers

Cross ReactivityCross Reactivity Modification of a molecule; toxins and toxoidsModification of a molecule; toxins and toxoids

Sharing epitopes between unrelated macromoleculesSharing epitopes between unrelated macromolecules

Structural resemblance (molecular mimicry)Structural resemblance (molecular mimicry)

Significance in Significance in - tolerance and autoimmunity- tolerance and autoimmunity - Isohemagglutinins - Isohemagglutinins

Antigens: T-independentAntigens: T-independent Activate B cells Activate B cells withoutwithout MHC class II T help MHC class II T help PolysaccharidesPolysaccharides PropertiesProperties

Polymeric structurePolymeric structure Polyclonal B cell activation, but poor memoryPolyclonal B cell activation, but poor memory Resistance to degradationResistance to degradation

ExamplesExamples Pneumococcal polysaccharide, LPSPneumococcal polysaccharide, LPS FlagellaFlagella

Antigens: T-dependentAntigens: T-dependent Require T help to activate B cellsRequire T help to activate B cells ProteinsProteins StructureStructure ExamplesExamples

Microbial proteinsMicrobial proteins Non-self or altered-self proteinsNon-self or altered-self proteins

Hapten-carrier conjugatesHapten-carrier conjugates DefinitionDefinition

Ag only if bound to carrier proteinAg only if bound to carrier protein StructureStructure

Native determinantsNative determinants Haptenic determinantsHaptenic determinants

Sequential (or linear) determinantsSequential (or linear) determinants Epitopes formed by several adjacent amino acid Epitopes formed by several adjacent amino acid

residues are called linear determinants.residues are called linear determinants.

They exist on the surface of antigen molecules or They exist on the surface of antigen molecules or inside of antigen molecules.inside of antigen molecules.

They are mainly recognized by T cells, but some They are mainly recognized by T cells, but some can also be recognized by B cellscan also be recognized by B cells. .

ConformationalConformational determinantsdeterminants Conformational determinants are formed by amino Conformational determinants are formed by amino

acid residues that are not in a sequence but become acid residues that are not in a sequence but become spatially juxtaposed in the folded protein.spatially juxtaposed in the folded protein.

They normally exist on the surface of antigen They normally exist on the surface of antigen molecules.molecules.

They are recognized by B cells or antibody. They are recognized by B cells or antibody.

Antigenic DeterminantsAntigenic Determinants Recognized by B cells and Ab Recognized by B cells and Ab

CompositionComposition Proteins, polysaccharides, nucleic acidsProteins, polysaccharides, nucleic acids Sequence (linear) determinantsSequence (linear) determinants Conformational determinantsConformational determinants

SizeSize 4-8 residues4-8 residues

Antigenic DeterminantsAntigenic Determinants Recognized by B cells and Ab Recognized by B cells and Ab

CompositionComposition SizeSize NumberNumber

Limited (immunodominant epitopes)Limited (immunodominant epitopes) Located on the external surfaces of the AgLocated on the external surfaces of the Ag

Antigenic DeterminantsAntigenic DeterminantsRecognized by T cellsRecognized by T cells

CompositionComposition Proteins (some lipids)Proteins (some lipids) Sequence determinantsSequence determinants

ProcessedProcessed MHC presentation (lipid presentation by MHC-like CD1)MHC presentation (lipid presentation by MHC-like CD1)

SizeSize 8 -15 residues8 -15 residues

NumberNumber Limited to those that can bind to MHCLimited to those that can bind to MHC

T cell

TCR

T cell

TCR

APC

MHC

APC

MHC

Ag Super Ag

SuperantigensSuperantigens

Definition Polyclonal T cell

response Examples

Staphylococcal enterotoxins

Toxic shock toxin

SuperantigensSuperantigens

Conventional Antigen

Monoclonal/ Oligoclonal T cell response

1:104 - 1:105

Superantigen

Polyclonal T cell response

1:4 - 1:10

DefinitionDefinition

Most Important Human Most Important Human AntigensAntigens

Membrane molecules of immune cells Membrane molecules of immune cells

Receptors: TCR, BCR, CR, CKR, FcRReceptors: TCR, BCR, CR, CKR, FcR Class IClass I and class and class ⅡⅡ MHC molecules MHC molecules CD molecules: CD1~339CD molecules: CD1~339 Cell Adhesion MoleculesCell Adhesion Molecules Cytokine ReceptorsCytokine Receptors Blood Group AntigensBlood Group Antigens

Pathogen recognition by adaptive immunity: great Pathogen recognition by adaptive immunity: great variety, selectivityvariety, selectivity

T LymphocytesT Lymphocytes• Distinguishing cell-surface

markers include TCR, CD3, CD2, CD4 or CD8, CD28, and CD45

• Similarities between T and B cells:• Antigen receptor on surface (TCR)• Recognize single, specific

antigen• Expand through clonal selection• Some T cells exist as long-lived

memory cells

B LymphocytesB Lymphocytes• Recognize antigen by means of surface-expressed antigen receptor

• Distinguishing cell-surface markers include: B220 (CD45), MHC Class II, CD80 (B7-1) and CD86 (B7-2), CD40, CD19, CD21, etc.

Bursa offabricius

Figure 3-13 part 1 of 2Figure 3-13 part 1 of 2

Figure 3-15Figure 3-15The peptide-binding groove of MHC molecules

Present Ag to CD4 T cells

Present Ag to CD8 T cells

Help peptide loading

Present antigen peptidesto CD4+ T cells

Polymorphism allows the population to handle a variety of pathogens.

Polymorphism: presence of multiple alternative forms (alleles) of a gene.

Figure 3-22Figure 3-22• Almost all cells express MHC I for comprehensive surveillance by CD8 T cells

• Only some cells express high levels of MHC II and MHC I

• These are B cells, macrophages, dendritic cells and thymic epithelial cells.

• B cells, macrophages and dendritic cells are called professional antigen- presenting cells (APC).

• IFN- increases the expression of MHC II in APC and induces the expression in non-APC cells at sites of infection

Different cell distribution of MHC class I and II

Leukocyte Differentiation Antigens and CDLeukocyte Differentiation Antigens and CD

Leukocyte differentiation antigenLeukocyte differentiation antigen:: Cell Cell surface molecules expressed (or disappeared) surface molecules expressed (or disappeared) during different developmental and differential during different developmental and differential phases, activation or inactivation process of phases, activation or inactivation process of blood cells.blood cells.

Identifying Cell Using the CD Identifying Cell Using the CD NomenclatureNomenclature

CD Cluster Of DifferentiationCD Cluster Of Differentiation Over 300 CD MarkersOver 300 CD Markers T cells, CD4 or CD8 and CD3T cells, CD4 or CD8 and CD3 B cells, CD19B cells, CD19 NK cells, CD56NK cells, CD56 Monocytes /Macrophages CD14Monocytes /Macrophages CD14 Dendritic Cells, CD1cDendritic Cells, CD1c

Table 2-4CD - Cluster of Differentiation

CDs which take part in T cell recognition, CDs which take part in T cell recognition, adhesion and activationadhesion and activation

CDs which take part in B cell recognition, CDs which take part in B cell recognition, adhesion and activationadhesion and activation

Adhesion MoleculesAdhesion Molecules

Cell adhesion molecules (CAMs) Cell adhesion molecules (CAMs) are cell surface are cell surface proteins involved in the interaction of cell-cell or proteins involved in the interaction of cell-cell or cell-extracellular matrix.cell-extracellular matrix.

CAMs take effect by the binding of receptor and CAMs take effect by the binding of receptor and ligand. ligand.

ⅡⅡ. Classification. Classification

Integrin familyIntegrin family Selectin family Selectin family Immunoglobulin (Ig) superfamilyImmunoglobulin (Ig) superfamily Cadherin family Cadherin family Mucin - like familyMucin - like family Other adhesion moleculesOther adhesion molecules

1. Integrin family1. Integrin family Integrins consist of α and β chains.Integrins consist of α and β chains. According to β subunits, Integrins are divided into According to β subunits, Integrins are divided into

eight groups: β1- β8eight groups: β1- β8 VLA-4(VLA-4(VVery ery LLate ate AAntigen-4)------VCAM-1 ntigen-4)------VCAM-1 LFA-1(LFA-1(LLymphocyte ymphocyte FFunction-associated unction-associated AAntigen-1) ntigen-1)

ICAM-1,2,3 ICAM-1,2,3 MAdCAM-1 (Mucosal Addressin Cell Adhesion MAdCAM-1 (Mucosal Addressin Cell Adhesion

Molecule-1)Molecule-1) TSP-1 ((ThrombospondinTSP-1 ((Thrombospondin 一一 1)1)

2. Selectin family2. Selectin family

Selectins consist of one peptide chain.Selectins consist of one peptide chain.

The three family members include: The three family members include: E- selectinE- selectin, , L-selectinL-selectin, and , and P-selectinP-selectin..

3. Ig superfamily(IgSF)3. Ig superfamily(IgSF)

The structure of these adhesion molecules resemble The structure of these adhesion molecules resemble that of Ig.that of Ig.

CD4, CD8, CD2(LFA-2), CD58(LFA-3), VCAM-1, CD4, CD8, CD2(LFA-2), CD58(LFA-3), VCAM-1, ICAM-1,2,3ICAM-1,2,3

4. Cadherin family4. Cadherin family

E- cadherin------ Epithelia cellE- cadherin------ Epithelia cell N- cadherin------ Nerve cellN- cadherin------ Nerve cell P- cadherin-------PlacentaP- cadherin-------Placenta

5. Mucin -like family5. Mucin -like family CD34, GlyCAM-1(glycosylation dependent cell CD34, GlyCAM-1(glycosylation dependent cell adhesion molecule-1) adhesion molecule-1) PSGL-1(P-selectin glycoprotein ligand-1) PSGL-1(P-selectin glycoprotein ligand-1)

6. Other adhesion molecules6. Other adhesion molecules CD44CD44

ⅢⅢ. Functions. Functions1. Participate in development and

differentiation of immune cells CD2----LFA-3 LFA-1----ICAM-1 Participate in development and maturation of

thymocytes.2. Participate in immune response and regulation

IL-21

IL-10

Cytokine Receptor FamiliesCytokine Receptor Families

TLRs