1. بسم الله الرحمن الرحيم

2. Antigen Samira Rajaei, MD, PhD. Assistant professor Department of Immunology Tehran University of Medical Sciences

3. Antigen definition AntigenAnti(body)+ generator Antigen Immunogen Tolerogen

4. Which cells are the main players of adaptive immunity?

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

6.  Antibodies can recognize as antigens almost every kind of biologic molecule:  Lipids  Carbohydrates  Proteins  Nucleic acids  TCRs mainly recognize peptides Difference between Ag recognition

7. Immunogen Antigens are capable of activating lymphocyte

8. All antigens are recognized by specific lymphocytes ① true ②false All antigens are capable of activating lymphocyte ① true ②false True/False

9.  Foreign antigens may be administered in ways that preferentially induce tolerance rather than immune responses.  These antigens are tolerogen. Tolerogen

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

11. Immunogenicity Foreignness Molecular size Chemical structure complexity Dosage, Route, individual difference, timing of administration

12.  Foreignness is essential to immunogenicity  Self-responsive cells are eliminated during lymphocyte ontogeny  More Foreignness; more immunogenicity Foreignness

13.  >100000 Dalton  <10000 Dalton Molecular size

14.  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

15.  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

16.  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

17. How to generate antibody against haptens?

18.  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

19.  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

20.  Soluble : (proteins, polysaccharides) + Adjuvants  Particulate: bacteria, virus and RBCs (Adjuvants are not necessary)  Colloidal (non robust immunogens) Physicochemical structure of Antigens

21.  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?

22.  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

24. 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

25.  Most of the macromolecules have multiple determinants.

26. 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

27. 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.

28.  Nonoverlapping determinants  2 or more antibody bind to a protein antigen  No steric interference  Overlapping determinants  Steric interference Spatial arrangement of epitopes

29.  Allosteric effects (conformational changes positively or negatively effects another antibody binding without Steric interference) Allosteric Effects

30.  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

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

32.  Proteins may be subjected to modifications such as glycosylation, phosphorylation, ubiquitination, acetylation, and proteolysis.  Modifications produce new epitopes. Neoantigenic determinants

34.  T lymphocytes recognize linear peptide determinants in combination with MHC residues. Epitopes determined by T cells

35.  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

36.  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