Antigens, haptens and immunogens

Definitions:

An antigen is any substance that react with T or B lymphocytes

simple to macromolecules e.g. carbohydrates, phospholiplids, nucleic acids and proteins.

Immunogens are substances that generate immune response

Haptens are small molecules which could never induce an immune response unless coupled to a carrier molecule (foreign protein)

e.g. dinitrophenyl, aminobenzene sulphonate, arsonate

Haptein-carrier molecule, unlike free haptein, can acts as an immunogen.

Antibody-Antigen Interactions

Binding of antibody to antigen is dependent on hydrogen bonds, electrostatic attractions and Van der

Waals attractions.

These bonds are weak compared to covalent bonds but the large number of weak bonds result in a stable complex.

Antibody-antigen binding is reversible.

What is an Epitope

An epitope is the small site on the antigen which is recognized by the antibody.

Usually between one and six sugars or amino acids on the surface of the antigen.

Antibody Uniqueness: B-cells produce somewhere between 1 x 108

and 1 x 1010 IgG antibodies with different binding sites.

Antigenic determinant is a cluster of epitopes on the surface of an antigen.

Antigen has several determinants each structurally different from each other.

A monoclonal antibody reacts with only one determinant on the same antigen.

The affinity or strength of binding of antigen and antibody depends on closeness of fitting.

It refers to strength of association between an individual epitope and a paratope high affinity antibodies bind strongly to antigen.

Avidity is the strength of binding two molecules or cells to one another at multiple sites.

It is determined by heterogeneity of antibodies in serum heterogeneity of antigenic determinants ‘bonus effect’ = k avid = k1 x k2

Avidity – measure of the functional affinity of an antiserum for the whole antigen.

The specificity of antigen recognition by an antibody is not absolute

Cross-reactivity is due to antiserum reacting with a partially related antigen.

Monoclonal antibodies are directed towards a single epitope less cross – reactivity.

Factors Influencing ImmunogenicityContribution of the Immunogen

Foreignness Size

Conformational determinants

Sequence determinants

• Chemical Composition– Primary Structure– Secondary Structure– Tertiary Structure– Quarternary Structure

Factors Influencing ImmunogenicityContribution of the Immunogen

Foreigness Size Chemical Composition Physical Form

Particulate > Soluble Denatured > Native

Factors Influencing ImmunogenicityContribution of the Immunogen

Foreigness Size Chemical Composition Physical Form Degradability

Ag processing by Ag Presenting Cells (APC)

Factors Influencing ImmunogenicityContribution of the Biological System

Genetics Species Individual

Responders vs Non-responders

Age

Factors Influencing ImmunogenicityMethod of Administration

Dose Route

Subcutaneous > Intravenous > Intragastric Adjuvant

Substances that enhance an immune response to an Ag

Chemical Nature of Immunogens

Proteins Polysaccharides Nucleic Acids Lipids

Some glycolipids and phosopholipids can be immunogenic for T cells and illicit a cell mediated immune response

Types of AntigensT-independent

Polysaccharides Properties

Polymeric structure Polyclonal B cell

activation Yes -Type 1 (TI-1) No - Type 2 (TI-2)

Resistance to degradation

Examples Pneumococcal polysaccharide, lipopolysaccharide Flagella

Types of AntigensT-dependent

Proteins

Structure Examples

Microbial proteins Non-self or

Altered-self proteins

What Does The B Cell Immunoglobulin (Ig) Receptor Recognize?

1. Proteins (conformational determinants, denatured or proteolyzed determinants)

2. Nucleic acids3. Polysaccharides4. Some lipids5. Small chemicals (haptens)

What Does the αβ T Cell Receptor (TCR) Recognize?

1. Only fragments of proteins (peptides) associated with MHC molecules on surface of cells

Helper T cells (Th) recognize peptide associated with MHC class II molecules

Cytotoxic T cells (Tc) recognize peptide associated with MHC class I molecules

Antigenic Determinants Recognized by B cells and Ab

Composition Proteins, polysaccharides, nucleic acids Sequence (linear) determinants Conformational determinants

Size 4-8 residues

Antigenic Determinants Recognized by B cells and Ab

Composition Size Number

Limited (immunodominant epitopes)

Located on the external surfaces of the Ag

Fe

Antigenic DeterminantsRecognized by T cells

Composition Proteins (some lipids) Sequence determinants

Processed MHC presentation (lipid presentation by MHC-like

CD1)

Size 8 -15 residues

Number Limited to those that can bind to MHC

Conventional Antigen

αC βC

CHO CHO

CHOCHO

βVαV

α2 β2

β1α1CHO CHO

CHO

αC βC

CHO CHO

CHOCHO

βVαV

α2 β2

β1α1CHO CHO

CHO

MHC Class II

T cell receptor

AntigenSuper

antigen

T lymphocyte

Antigen presenting cell

Superantigen

SuperantigensProteins produced by pathogensNot processed by antigen presenting

cells Intact protein binds to variable region

of β chain on TCR of T cells and to MHC class II on antigen presenting cells (APC)

Large numbers of activated T cells release cytokines having pathological effects

Superantigens Definition Examples

Staphylococcal enterotoxins Staphylococcal toxic shock toxin Staphylococcal exfoliating toxin Streptococcal pyrogenic exotoxins

Antigen presenting cells (APC) Cells with the capacity to capture, process

and present antigenic peptides to T cells Antigens are presented in the context of

MHC class I or II Also deliver co-stimulatory signal (signal II)

to T cells leading to proper activation Only APCs can activate a naïve T cell

Dendritic cells, Macrophages, B cells

Pathway of Antigen Presentation by Antigen-Presenting Cells

CD4+ T cells T cells with CD4 marker (glycoprotein) 70% of T cells in the periphery T helper cells Play central role in modulating cellular immunity

via secretion of cytokines that modulate: B cell activation Immunoglobulin secretion (quality) Macrophage and dendritic cell activation Cellular chemotaxis and inflammation

Th1 versus Th2 cells

Differences b/w Th1 and Th2 cells

Produces type 1 cytokines IL-2, IFN-, TNF-,

TNF- Activates macrophages

and DCs for intracellular killing of pathogens

Mediates CMI

Produces type 2 cytokines IL-4, IL-5, IL-10, IL-13

Provides help to B cells in antibody response

Mediates allergy and immunity to extracellular pathogens, including parasites

Th1 cell Th2 cell

T cells T cell receptors (TCR) – Ag specific Glycoproteins CD4 (helper T cells) or CD8

(cytotoxic T cells)

CD4 T cell or helper T cell

CD8 T cell or cytotoxic T cell

CD4 CD8

TCR TCR

CD4+ T cells Helper T cells involved in Ab production Recognition of “exogenous Ag”

Bacteria Extracellular Ag

Recognize MHC class II molecule Present on “antigen presenting cells” = APC e.g. Macrophages, Dendritic Cells, B cells

CD8+ T cells

Cytotoxic T cells cell killing Recognition of “endogenous Ag”

Virus infected cells Cancerous cells

Recognize MHC class I molecule Present on all cells

All T cells are “Antigen specific”

Mediated by “T cell receptor” TCR Surface molecule analogous

to part of AbDiversity is generated by

rearrangement of TCR gene locus

T cell Membrane

T Cell Receptor

Antigen Recognition

Complement Binding Site

Phagocyte Receptor Binding Site

Antibody Structure

Figure 13.3, p. 251

TCR like this portion of Ab

TCR Recognizes its Epitope Only in the Context of MHC

CD4 TCR – peptide/MHC Class IICD8 TCR – peptide/MHC Class I

APC = e.g., Macrophage

TCR

PeptideAntigen

CD4

CD4 T Cell

MHCClass II

MHC Class I and Class II MHC I on all cells MHC II on APC Bind Ag only small peptides As an individual you make a small number of

different kinds of MHC I and MHC II Encoded by stable genes inherited; NOT

generated by rearrangements But in the population there are lots of genetic

variants of MHC I and MHC II Important in transplants Hence the name “Major Histocompatability Complex”

Cytotoxic T cells T cells that express CD8 molecule on their surface 30% of T cells in the periphery Destroy cells infected by intracellular pathogens and cancer cells Class I MHC molecules (nucleated body cells) expose foreign proteins TC cell releases perforin and granzymes, proteins that form pores in the target

cell membrane; causing cell lysis and/or apoptosis

Humoral response: B cells Stimulated by T-dependent antigens

(help from TH cells)

APCs with class II MHC proteins process and present antigen to CD4+ T cells (helper cells)

Helper T cell become activated

Activated T cell secretes cytokines that in turn activate B cell

B cell differentiates into effector and memory (plasma) cells and produce antibodies

Antibody-mediated effector mechanisms

Points Concerning Antigen Processing and Presentation1. Location of pathogen viruses in cytosol, MHC class I pathway,

Tc response

extracellular bacteria, MHC class II pathway, Th2 response, Ab formation

intracellular bacteria, MHC class II pathway, Th1 response

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