Dr. Nabil MTIRAOUI, M.Sc, Ph.DLecture 7

Antigens and Antigens and ImmunogensImmunogens

Properties and Characteristics

Definition and Properties

Introduction

In 1899 Ladislas Deutsch named the hypothetical substances halfway between bacterial constituents and antibodies "substances immunogens or antigens". He originally believed those substances to be precursors of antibodies, just like zymogen is a precursor of zymase. But by 1903 he understood that an antigen induces the production of immune bodies (antibodies) and wrote that the word antigen was a contraction of "Antisomatogen.

Immunogens are substances that generate immune response

Antigenicity is the ability to combine specifically with the final products of the immune responses ( antibodies and/or cell surface receptors).

Definition of Immunogen and Antigenicity

Antigen is substance which when introduced parentally into the body stimulates the production of an antibody with which it reacts specifically and in an observable manner

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

Definition of Antigen

Haptens are small molecules which could never induce an immune response unless coupled to a carrier molecule.

e.g. dinitrophenyl, aminobenzene sulphonate, arsonate

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

Definition of Hapten

Definition of 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.

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.

Definition of Epitope

Definition of Epitope

Substance, which when mixed with an antigen, enhances the magnitude and duration of the immune response

Functions:

1. Prolong retention of immunogen

2. Increase the effective size of the immunogen

3. Stimulate local influx of macrophages or immune cells to the injection site

Definition of Adjuvant

Classification of Antigens

Classification of Ag : Based on Immunogenicity

Complete antigen : Substances which can induce antibody formation by themselves and can react specifically with these antibodies

Incomplete antigen (haptens): substances unable to induce antibody formation on its own but can become immunogenic when covalently linked to proteins, called carrier proteins.

Exogenous antigens Exogenous antigens are antigens that have

entered the body from the outside, for example by inhalation, ingestion, or injection. The immune system's response to exogenous antigens is often subclinical.

Endogenous antigens Endogenous antigens are antigens that have

been generated within previously normal cells as a result of normal cell metabolism, or because of viral or intracellular bacterial infection.

Classification of Ag : based on origin

Auto-antigensAn autoantigen is usually a normal protein or complex of proteins (and sometimes DNA or RNA) that is recognized by the immune system of patients suffering from a specific autoimmune disease. These antigens should, under normal conditions, not be the target of the immune system, but, due to mainly genetic and environmental factors, the normal immunological tolerance for such an antigen has been lost in these patients.

Classification of Ag : based on origin

Types of Antigens

Types of Antigens

1. T-independent Antigens

• Can directly stimulate B cells to produce antibodies

• Polysaccharides in general

• Generally more resistant to degradation persist for longer periods of time continue to stimulate immune system

2. T-dependent Antigens

• Do not directly stimulate antibody production; need help of T cells

• Usually proteins

T-independent Antigen

Polysaccharides Properties

Polymeric structure Polyclonal B cell activation Resistance to degradation

Examples Pneumococcal polysaccharide, lipopolysaccharide Flagella

Proteins

Structure Examples

Microbial proteins Non-self or Altered-self proteins

T-dependent Antigen

Types of Antigens

Bacterial Antigens: these may be: Soluble Ags: which are products excreted into the

environment e.g. exotoxins, enzymes, haemolysin, ect.. Cellular Ags: which correspond to the different structure

of the bacterial cell e.g. Capsular Ags in capsulated organisms. Flagellar or H Ags in flagellated(motile)organisms. Somatic or O Ags are parts of the cell wall of gram

negative bacteria. Virulence or Vi Ags are surface Ags. Fimbrial Ags are surface Ags in fimriated gram negative

bacteria.

Viral Antigens: these may be:a) Protein coat viral Ags.b) Soluble Ags: which diffuse in the surrounding

fluids during virus growth e.g. soluble nucleoprotein as in influenza and mumps viruses.

Types of Antigens

Human tissue antigens (isoantigens):a) Blood group Ags: A and B as well as Rh Ags on red

cells. These are important in blood transfusion reactions.

b) Histocompatibility Ags: these are glycoprotein molecules present on membranes of tissue cells. They are called the major histocompatibility complex (MHC) Ags or human leucocyte Ags (HLA). There are 2 classes of MHC: class1 MHC are present on all nucleated cells, class2 MHC are present on immunocompetent APC.

Types of Antigens

Antibody-Antigen Interactions

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.

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

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

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

Antibody-Antigen Interactions

Antibody-Antigen Interactions: Affinity

Affinity = attractive and repulsive forces

Ab

Ag

High Affinity

Ab

Ag

Low Affinity

• Strength of the reaction between a single antigenic determinant and a single Ab combining site

• The overall strength of binding between an Ag with many determinants and multivalent Abs

Keq = 104

Affinity106

Avidity1010

Avidity

Antibody-Antigen Interactions: Avidity

• The ability of an individual Ab combining site to react with more than one antigenic determinant.

• The ability of a population of Ab molecules to react with more than one Ag

Anti-A Ab

Ag A

Anti-A Ab

Ag B

Identical epitope

Anti-A Ab

Ag C

Similar epitope

Cross reactions

Antibody-Antigen Interactions: Cross Reactivity

Properties of Antigens

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

What Does The B Cell Ig Receptor Recognize?

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

2. Nucleic acids3. Polysaccharides4. Some lipids5. Small chemicals

(haptens)

What Does the αβT Cell Receptor (TCR) Recognize?

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

Th recognize peptide associated with MHC class II molecules

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

Number Limited (immunodominant epitopes) Located on the external surfaces of

the Ag

Fe

Antigenic Determinants : Recognized 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

Superantigens

Proteins produced by pathogens

Not 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

Definition Examples

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

Superantigens

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

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”

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

All T cells are “Antigen specific”

Mediated by “T cell receptor” TCR Surface molecule analogous to part of Ab

Diversity is generated by rearrangement of TCR gene locus

TCR Recognizes its Epitope Only in the Context of MHC

CD4 TCR – peptide/MHC Class II CD8 TCR – peptide/MHC Class I

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

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

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