Antigens PDF

Antigens Definition These are any foreign substance which can stimulate the immune system  Epitopes or Antigenic Determinants  Specific small parts of the antigenic molecules which can stimulate the specific immune system  Composed of 4-7 amino acids or monosaccharid...

Antigens Definition These are any foreign substance which can stimulate the immune system  Epitopes or Antigenic Determinants  Specific small parts of the antigenic molecules which can stimulate the specific immune system  Composed of 4-7 amino acids or monosaccharide residues  Factors influencing antigenicity: 1- Foreignness: 2- Molecular size: 3- Chemical nature: The immune system of an The most potent antigens Small molecules are individual can are proteins  usually not normally distinguish polysaccharides of immunogenic e.g. between body high molecular weight amino acids components (self) and foreign substances (non– self)  Factors influencing antigenicity: 4- Methods of administration of antigens: Dosage amount: Dosage number: Route of administration: if very high or very low Repeated booster doses Parentral routes ˃ oral doses  specific  strong immune routes as they induce  unresponsiveness response stronger immune (immunologic tolerance) response Adjuvant: - Non-antigenic substance when mixed with an antigen before its administration  increase the immune response to that antigen - e.g. alum precipitated toxoid in Diphtheria vaccine - slows antigen absorption  more prolonged stimulation of the immune cells Factors influencing antigenicity: 5- Host genetic factors: All individuals within a species will not show the same response to an antigen; some are strong responders and others are weak responders Haptens: Substances of low molecular weight which are not capable of inducing an immune response by themself. Hapten + larger carrier proteins or bind to tissue proteins  immunogenic and capable of inducing an immune response Hapten  antigenic determinant on the carrier molecule e.g. penicillin, sulphonamides, neomycin, skin ointments, cosmetics and soap Types of antigens: 1. Bacterial antigens 2. Viral antigens 3. Human tissue antigens (isoantigens) 4. Heterophil antigen 5. Superantigens  Types of antigens: 1. Bacterial antigens a. Soluble antigens which are 2. Viral antigens products excreted into the 3. Human tissue antigens environment e.g. exotoxins, 4. Heterophil antigen enzymes, etc. 5. Superantigens b. Cellular antigens e.g. capsular antigens, flagellar or "H" antigens, somatic or "O" antigens which are polysaccharide part of the cell wall of Gram-negative bacteria.  Types of antigens protein coat viral 1. Bacterial antigens antigens. Antibodies against these surface 2. Viral antigens components can neutralize viral infectivity. 3. Human tissue antigens (isoantigens) 4. Heterophil antigen 5. Superantigens Types of antigens 3. Human tissue antigens (isoantigens) Blood group antigens: A and B + Rh antigens on red cells Types of antigens 3. Human tissue antigens (isoantigens) Histocompatability antigens: - Molecules present on the membrane of tissue cells - human leukocyte antigens (HLA) or major histocompatibility complex (MHC) antigens - Types: Class I: present on all nucleated cells Class II: present on antigen presenting cells (APCs) e.g. B cells, macrophages, dendritic cells and activated T cells  MHC significance: 1. Organ transplantation and graft rejection 2. MHC restriction 3. Paternity testing and forensic investigations 4. Disease association: the presence of certain HLA antigens is often associated with a particular disease e.g. HLA-DR4 with rheumatoid arthritis Types of antigens 4.Superantigens  They are antigens that are capable of activating large numbers of T cells.  They are produced by many pathogens but most important are staphylococcal enterotoxins, toxic shock syndrome toxins and others.  They bind class II MHC molecules (outside of the peptide binding groove) and the T cell receptor (TCR) - β chain (variable region only)  They cause a large percent of T cells to be stimulated causing the release of large amounts of cytokines.  This cause systemic toxicity and suppression of immune response Types of antigens 4.Superantigens Antigen Super-antigen Processed in APC Not processed Bind to both α andβ chains Bind to β chain only Specific activation of T cells Non specific Release the required level of Very high harmful levels are cytokines released Classic Ag Vs Super Ag Types of antigens 5. Heterophil antigen: When the reacting substances are unrelated but share some of the same epitopes These are known as heterophil antibodies and heterophil antigens and the phenomenon is known as "cross reactivity"  Antigen antibody binding: binding of antigen to antibody binding site is like a “lock and key” situation  Antigen antibody binding: Acute rheumatic fever Antibodies against Strept. pyogenes (M antigen) cross react with cardiac muscle proteins  rheumatic carditis and valve destruction 1.Substances that are immunologic only when coupled to a protein carrier are called: a) Adjuvants b) Epitopes c) Haptens d) Opsonins 2.The site on the antigen which will elicit immune response and react with antibody is:. a) Immunogen. b) Agglutinin. c) Epitope. d) Precipitin Which of the followings is an example of Isoantigen ? a) Bacterial capsule. b) Blood groups. c) Capsid. d) Toxins.