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Antigens.pptx
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- 2. INTRODUCTION Antigens arelarge molecules of proteins Present on the surface of the pathogen- such as bacteria, fungi viruses, and other foreign particles. When these harmful agents enter the body, it induces an immune response in the body for the production of antibodies. For example: When a common cold virus enters the body, it causes the body to produce antibodies to prevent from getting sick.
- 3. Specificity willbe regulated due to recognition of antigenic determinants or epitopes on the surface of the Antigen Epitopes = immunologically active regions that bind to: Ag-specific membrane receptors on lymphocytes Already secreted antibodies Many epitopes can be found on single molecule INTRODUCTION
- 4. PROPERTIES OF ANTIGENS The antigen should be a foreign substance to induce an immune response. The antigens have a molecular mass of more than 5000 Da. They are mainly proteins and polysaccharides. The more chemically complex they are, the more immunogenic they will be. Antigens are species-specific. The age influences the immunogenicity. Very young and very old people exhibit very low immunogenicity.
- 5. TYPES OF ANTIGENS BASEDON ORIGIN Exogenous Antigens Endogenous Antigens Autoantigens Tumour Antigens
- 6. EXOGENOUS ANTIGENS Exogenousantigens are the external antigens that enter the body from outside.E.g. inhalation, injection, etc. This include food allergen, pollen, aerosols, etc. and are the most common type of antigens. ENDOGENOUS ANTIGENS Endogenous antigens are generated inside the body due to viral or bacterial infections
- 7. AUTO ANTIGENS Autoantigensare the ‘self’ proteins or nucleic acids that due to some genetic or environmental alterations get attacked by their own immune system causing autoimmune diseases. TUMOUR ANTIGENS It is an antigenic substance present on the surface of tumour cells that induces an immune response in the host. Many tumors develop a mechanism to evade the immune system of the body. Native Antigens An antigen that is not yet processed by an antigen-presenting cell is known as native antigens.
- 8. ATTRIBUTES OF ANTIGENECITY INDUCTION OF IMMUNE RESPONSE – IMMUNOGENECITY SPECIFIC REACTION WITH ANTIBODIES - IMMUNOLOGICAL REACTIVITY An antigen introduced in the body specifically reacts with those particular immunocytes (B or T Lymphocytes) which carry unique markers which produce specific antibodies or complementary cells. Antibody such produced will react with only that particular Antigen.
- 9. BASED ON IMMUNERESPONSE Antigens can be classified as: IMMUNOGEN OR COMPLETE ANTIGEN These may be proteins or polysaccharides and can generate an immune response on their own. Ability to produce observable and specific activity with the antibody HAPTEN These are non-protein, foreign substances that require a carrier molecule to induce an immune response.
- 10. Immunogenicity =ability to induce humoral and/or cell- mediated immune response Antigenicity = ability to combine specifically with the final products of the above responses (i.e., antibodies and/or cell-surface receptors). Haptens, small molecules, are antigenic but incapable of inducing specific immune response Immunogenicity and Antigenicity
- 11. ANTIGENS VS IMMUNOGENS An immunogen refers to a molecule that is capable of producing an immune response by the immune system. An antigen refers to a molecule that is capable of binding to the product of that immune response. So, an immunogen is necessarily an antigen, but an antigen may not necessarily be an immunogen. Vaccines are examples of antigens in an immunogenic form, which are intentionally administered to a recipient to induce the memory function towards antigens of the pathogen invading that recipient
- 12. STRUCTURE OF ANTIGENS The epitopes or antigenic determinants are the components of antigen. Every antigen has several epitopes. An antibody has at least two binding sites that can bind to specific epitopes on antigens. The antigens combine with the antibody according to the lock and key mechanism. The ability of the body to act against the disease-causing agents and antigens by the immune system is termed as the immunity. This immunity may be either inborn or acquired from vaccinations.
- 13. EPITOPE SMALLEST AREAON THE ANTIGEN MOSTLY CONSIST OF 4 OR 5 AMINOACIDS OR A MONOSACCHARIDE POSSESS A SPECIFIC CHEMICAL STRUCTURE ELECTIRCAL CHARGE SPATIAL CONFIGURATION CAPABLE OF SENSITISING AN IMMUNOCYTE AND REACT WITH THE COMPLIMENTARY SITE ON THE ANTIBODY OR T CELL RECEPTOR IT MAY BE LINEAR SEQUENTIAL SEGMENT OR BRINGING TOGETHER RESIDUES FROM DIFFERENT SITES OF THE PEPTIDE MAKING A CONFORMATIONAL EPITOPE.
- 14. IMMUNE CELLSDO NOT INTERACT WITH, OR RECOGNIZE, ENTIRE IMMUNOGEN MOLECULE LYMPHOCYTES RECOGNIZE DISCRETE SITES ON IMMUNOGEN MOLECULE CALLED EPITOPES, OR ANTIGENIC DETERMINANTS. EPITOPES ARE THE IMMUNOLOGICALLY ACTIVE REGIONS OF AN IMMUNOGEN THAT BIND TO ANTIGEN-SPECIFIC MEMBRANE RECEPTORS ON LYMPHOCYTES OR TO SECRETED ANTIBODIES STUDIES WITH SMALL ANTIGENS REVEAL THAT B AND T CELLS RECOGNIZE DIFFERENT EPITOPES ON SAME ANTIGENIC MOLECULE EPITOPE
- 15. RECOGNITION OF EPITOPES T CELL RECOGNISES SEQUENTIAL EPITOPE B CELL RECOGNISES TERTIARY STRUCTURE OF THE CONFORMATIONAL EPITOPE THE COMBINING AREA ON THE ANTIBODY CORRESPONDING TO THE EPITOPE IS CALLED THE PARATOPE EPITOPE AND THE PARATOPE DETERMINE THE SPECIFICITY OF THE IMMUNOLOGICAL REACTIONS ANTIGENS ON THE BACTERIA AND VIRUSES CARRY DIFFERENT TYPES OF EPITOPES PRESENTING AN ANTIGENIS MOSAIC. PRESENCE OF THE SAME OR DIFFERENT EPITOPES ON DIFFERENT ANTIGENS ACCOUNTS OF ANTIGENIC CROSS REACTIONS.
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- 18. ANTIGEN AND ANTIBODYREACTION
- 19. BASIC PROPERTIES AREKNOWN TO BE ESSENTIAL TO MAKE A ANTIGENS Foreign Nature Molecular Size Antigenic Determinants and Cross-Reactivity Molecular Rigidity and Complexity Genetic Factors in the Individual.
- 20. FOREIGN NATURE Theimportant requirement for immunogenicity is that the substance should be foreign to the body of recipient. With result the immune system must be able to distinguish between normal body components and foreign substances. Normally the body has no tendency to recognize its own components as antigens, therefore does not initiate an immune response against these. For example, if serum albumin from a mouse is injected back into the same mouse or into another mouse, no antibody production will occur. On the other hand, if the albumin is infected into an animal of another species, antibody may be produced.
- 21. MOLECULAR STRUCTURES Thesubstance or molecule must have certain physio-chemical properties, where molecular size is an important factor. It is established that a molecule must have a minimum size (generally > 5000 Da) before it can be considered as antigen. Hence small molecules like amino acids or monosaccharides are usually not antigenic. However, low molecular substance can demonstrate immunogenicity, if coupled to a suitable carrier molecule like protein. These low molecular substances are called haptens which are considered as ‘partial antigens’ that contain at least one of the determinant groups of an antigen. Hapten can thus reacts with specific antibody but is not immunogenic in absence of a carrier molecule. Example of Haptens: Drugs, peptide hormones, and steroid hormones
- 22. Partial antigens orHaptens
- 23. ANTIGENIC DETERMINANTS Onlylimited parts of the large antigen molecule are involved in the binding to antibodies. These parts are called antigenic determinants. It is established that a molecule must have at least two antigenic determinants in order to stimulate antibody production. For this reason a small molecule does not function as antigen. Because it is not possible for a small molecule to have more than one antigenic determinant. The number of antigenic determinants on a small molecule varies with molecular size. Say for example, human serum albumin (Mol. wt.70 KDa) has at least six different antigenic determinants, that means at least six antibodies of different specificities can be produced after immunization. It is established that antibodies recognize the three dimensional structure of the antigen rather than their primary amino acid sequence.
- 24. CROSS-REACTIVITY In somecases antibodies can bind to an antigen other than one which initiated antibody production. This is known as cross-reactivity. For example, the homologous ‘antigen a’ has induced production of the antibody specific for antigen a and cross-reacts with non-identical ‘antigen b’ (Fig.10.2). Through cross-reactivity an allergic reaction can be initiated by a substance with which the individual has never been in contact before.
- 25. RIGIDITY AND COMPLEXITY The firm and stiff nature and complexity of the molecule are the important factors for immunogenicity. A rigid molecule is a good antigen, probably because it is easier to raise antibodies to certain structures than to others. The immunogenicity is very much dependent on the complexity of the antigens. For example, a molecule containing a repeating unit of only one amino acid is generally poor immunogen, even if the molecule is large; while a molecule with two or three repeating amino acids can, however, function well as an immunogen.
- 26. GENETIC FACTORS INTHE INDIVIDUAL Genetic factors are of prime importance for the immunogenicity. It was Wyman (1872) who for the first time reported that allergy runs in families and some families suffer more than others. All individuals within a species need not show the same allergic response against a particular antigen. Anyone can develop an allergy but the probability is increased if one or both the parents suffer from some kind of allergic condition. The highest frequency of hypersensitivity among children is observed where both parents are atopic and with same manifestation. So the frequency of allergy among children is progressively increased according to the nature and intensity of allergy in parents. It is evident that about 10-20% of the population suffer from aeroallergens such as pollen grains and spores.