بسم الله الرحمن الرحيم

Antigen

Samira Rajaei, MD, PhD.Assistant professor

Department of ImmunologyTehran University of Medical Sciences

Antigen definition

Antigen Anti(body)+ generator

• Antigen

• Immunogen

• Tolerogen

Which cells are the main players of adaptive immunity?

Any substance that may be specifically recognized by an antibody molecule (BCR) or T cell receptor

(TCR)

Antigen

Antibodies can recognize as antigens almost every kind of biologic molecule: Lipids Carbohydrates Proteins Nucleic acids

TCRs mainly recognize peptides

Difference between Ag recognition

Immunogen

Antigens are capable of activating lymphocyte

All antigens are recognized by specific lymphocytes ① true ②falseAll antigens are capable of activating lymphocyte① true ②false

True/False

Foreign antigens may be administered in ways that preferentially induce tolerance rather than immune responses.

These antigens are tolerogen.

Tolerogen

Small chemicals (dinitrophenol) may bind to antibodies, and are therefore antigens, but cannot activate B cells on their own (they are not immunogenic)

Hapten

Immunogenicity

Foreignness Molecular sizeChemical structure

complexityDosage, Route, individual difference, timing of

administration

Foreignness is essential to immunogenicity

Self-responsive cells are eliminated during lymphocyte ontogeny

More Foreignness; more immunogenicity

Foreignness

>100000 Dalton <10000 Dalton

Molecular size

Composition Proteins (most potent) Polysaccharides (second potent) Nucleic acids and lipids (are not immunogenic

unless …) Chemical complexity

Homopolymrs of amino acids? Which one of protein structures are more

immunogen? More complex molecules are more immunogen

Chemical structure complexity

High doses of immunogen may cause a lack of responsiveness (tolerance)

Intermediate doses of immunogen are generally the best immune response inducers

Low doses of immunogen may not induce immune responses

Dosage

Route of administration influences which cells (and how many of them) are stimulated s.c. (subcutaneous) i.d. (intradermal) i.m. (intramuscular) i.v. (intravenous) Oral administration i.n. (intranasal)

Route of immunogen administration

How to generate antibody against

haptens?

attach them to a protein before immunization which is called carrier.

attaching a number of hapten molecules to a single molecule of a polysaccharide (render it multivalent)

Hapten+Carrier

An adjuvant is any substance that enhances the immunogenicity of substances mixed with it

Many adjuvants in experimental use are microbial products, such as killed mycobacteria and LPS, that engage TLRs

What are the differences between Adjuvant and carrier? do not form stable linkages with the immunogen adjuvants are needed primarily for initial immunizations,

whereas carriers are required to elicit not only primary but also subsequent responses to haptens.

Adjuvant

Soluble : (proteins , polysaccharides) + Adjuvants

Particulate: bacteria, virus and RBCs (Adjuvants are not necessary)

Colloidal (non robust immunogens)

Physicochemical structure of Antigens

Delayed release of antigens Adjuvants convert soluble protein antigens into

particulate material, which is more readily ingested by antigen-presenting cells such as macrophages

adjuvants activate dendritic cells to express more MHC, increase the expression of costimulators, and cytokines needed for T cell activation, stimulate migration of the dendritic cells to lymph nodes .

How adjuvants enhance immunogenicity of proteins?

Freund’s adjuvant Incomplete (oil in water emulsion) Complete (oil in water emulsion and dead

mycobacteria)

Alum (aluminum hydroxide or aluminum phosphate)

Types of adjuvants

Epitope/Determinant

Macromolecules (proteins, polysaccharides, and nucleic acids) are usually much bigger than the antigen-binding region of an antibody molecule

Any antibody binds to only a portion of the

macromolecule, which is called

a determinant or an epitope

Most of the macromolecules have multiple determinants.

Polyvalency/multivalency

The presence of multiple identical determinants in an antigen is referred to as polyvalency or multivalency. Polysaccharides nucleic acids

Surface of microbs Polyvalent antigens can induce

clustering of the B cell receptors and initiate the process of B cell activation

Globular proteins

Most globular proteins do not contain multiple identical epitopes and are not polyvalent

Proteins could not cross link the B cell receptors.

They need the help of T lymphocytes. Proteins are T-dependent antigens.

Nonoverlapping determinants 2 or more antibody bind to a protein antigen No steric interference

Overlapping determinants Steric interference

Spatial arrangement of epitopes

Allosteric effects (conformational changes positively or negatively effects another antibody binding without Steric interference)

Allosteric Effects

Epitopes formed by several adjacent amino acid residues are called linear determinants.

The antigen-binding site of an antibody can usually accommodate a linear determinant made up of about 6 amino acids.

If linear determinants appear on the external surface or in a region of extended conformation in the native folded protein, they may be accessible to antibodies

More often, linear determinants may be inaccessible in the native conformation and appear only when the protein is denatured.

Linear determinants

Conformational determinants are formed by amino acid residues that are not in a sequence but become spatially juxtaposed in the folded protein.

Conformational determinants

Proteins may be subjected to modifications such as glycosylation, phosphorylation, ubiquitination, acetylation, and proteolysis.

Modifications produce new epitopes.

Neoantigenic determinants

T lymphocytes recognize linear peptide determinants in combination with MHC residues.

Epitopes determined by T cells

These molecules are able to bind to TCR complex and CD2, causing them to cluster on the cell surface, thereby mimicking the clustering caused by antigen presentation.

They activate the T cells regardless of their antigen specificity. Phytohemagglutinin (PHA), a lectin Concanavalin-A

Mitogens

Molecules that can activate T cells non-specifically

Superantigens bind to MHC class II molecules on APCs and are recognized by TCRs, but not in the same way as an MHC molecule-peptide complex.

Binding is to the Vβ chain of the TCR Staphylococcal enterotoxins Toxic shock syndrome toxin

Superantigens