Here is a term paper on ‘Antigens’ for class 11 and 12. Find paragraphs, long and short term papers on ‘Antigens’ especially written for school and college students.
Term Paper on Antigens
Term Paper # 1. Introduction to Antigens:
Adaptive immune responses generally arise as a result of the foreign compounds. The compound that generates the response is referred to as antigen. An antigen is an agent which is capable of binding specifically to components of the immune response, such as the B cell receptor (BCR) on B lymphocytes and soluble antibodies. The ability of an antigen to bind with antibodies and with cells of immune system is referred to as antigenicity.
Functional Distinction between the Antigen and Immunogens:
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The compound that generates the immune response is generally referred to as antigen; it is more appropriately called an immunogen and is therefore known as immunogenic. The difference between the two (antigen and immunogen) terms is necessary because there are certain compounds which are incapable of inducing an immune response, but then also they are capable of binding with components of the immune system that have been induced specifically against them.
Thus all immunogens are antigens, but all antigens are not immunogens. Some small molecules (haptens) are antigenic but incapable of inducing a specific immune response by them. Therefore, they lack immunogenicity. The study of antigen-antibody reactions in vitro is called serology. Serological reactions are the basis for all diagnostic immunology tests.
Term Paper # 2.
Requirements for Immunogenicity:
A substance must possess the following characteristics to be immunogenic:
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1. Foreignness:
The most important feature of an immunogen is that it must be foreign with respect to the host. The adaptive immune system recognizes and eliminates only foreign (non-self) antigens. Self-antigens are not recognized and thus individuals are tolerant to their own self molecules, even though these same molecules have the capacity to act as immunogens in other individuals of the same species.
2. Size:
The second requirement for a compound to be immunogenic is that the compound should have a certain minimal molecular weight. Small compounds with molecular weight < 1000 Da (e.g., penicillin, aspirin) are not immunogenic; those of molecular weight between 1000 and 6000 Da (e.g., insulin, adrenocorticotropic hormone) may or may not be immunogenic; and those of molecular weight > 6000 Da (e.g., albumin, tetanus toxin) are generally immunogenic. The most active immunogens tend to have a molecular mass of 100,000 Da or more. Therefore, small substances have less immunogenicity in comparison to large substances.
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3. Chemical Complexity:
The third characteristic to be immunogenic for a compound is certain degree of chemical complexity. For example, amino acids or sugars homo-polymers are seldom good immunogens regardless of their size. Similarly, a homo-polymer of poly-D-glutamic acid (the capsular material of Bacillus anthracis) with a molecular weight of 50,000 Da is not immunogenic. The absence of immunogenicity is because these compounds, although of high molecular weight, are not sufficiently chemically complex. Virtually all proteins are immunogenic.
Thus the most common immune responses are those to proteins. Furthermore, the greater the degree of complexity of the protein, the more vigorous will be the immune response to that protein. Nucleic acids are poor immunogens by themselves, but they become immunogenic when they are conjugated to protein carriers. Lipids are rarely immunogenic, but an immune response to lipids may be induced if the lipids are conjugated to protein carriers.
4. Dosage and Route of Administration:
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When there is insufficient dose of antigen it may not stimulate an immune response because the amount administered fails to activate enough lymphocytes or because such a dose renders the responding cells unresponsive.
Immunogens can be administered through a number of common routes:
i. Intravenous (into a vein) – Intravenously administered antigens are carried first to the spleen, where they can either induce immune unresponsiveness or tolerance, or if presented by APCs, (Antigen Presenting Cell) generate an immune response.
ii. Subcutaneous (beneath the skin) – Antigens which enters subcutaneously moves first to local lymph nodes.
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iii. Orally administered antigens (gastrointestinal route) elicit local antibody responses within the intestinal lamina propria.
iv. Intradermal (into the skin).
v. Intramuscular (into the muscle).
Antigens generally enter through the most common route namely, subcutaneous. It generally gives the strongest immune responses. This is due to their uptake, processing, and presentation to effector cells by Langerhans cells present in the skin, which are among the most potent APCs.
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Haptens:
Haptens are the substances which fail to induce immune responses in their native form because of their low molecular weight and their chemical simplicity. Haptens are antigenic but not immunogenic. Haptens become immunogenic only when they are conjugated to high molecular weight complex carriers. Therefore, hapten is a compound which is itself incapable of inducing an immune response but against which an immune response can be induced by immunization with the hapten conjugated to a carrier.
Term Paper # 3.
Affinity and Avidity of Antigens:
Affinity of Antigens:
It is the intrinsic constant association that characterizes the non-covalent interaction between single antigen binding sites of an antibody with an epitope. Low-affinity antibodies bind antigen weakly and tend to dissociate readily, whereas high-affinity antibodies bind antigen more tightly and remain bound longer.
When many repeating identical epitopes are present on the antigen or when antigens are multivalent, the entire association between the antigen molecules and antibodies not only depends on the affinity between each epitope and its corresponding antibody but also on the sum of the affinities of all the epitopes involved.
Avidity of Antigens:
Avidity refers to the overall binding between antibodies and a multivalent antigen. When complex Ag having multiple repeating epitopes is mixed with Ab having multiple binding sites, the interaction between multivalent Ab and Ag is called avidity. For example, Immunoglobulin, IgM has 10 antigen binding sites which are of higher avidity than IgG antibodies, although the binding of each Fab in the IgM antibody with ligand may be of the same affinity as that of the Fab from IgG.
Adjuvant:
An adjuvant is a substance which when mixed with an immunogen and injected with it, enhances or increases the immune response against the immunogen. Adjuvant is often used to boost the immune response in case of weak immunogens or when antigens are available in small amount. A carrier for a hapten and an adjuvant is different. Therefore, an adjuvant enhances the immune response to immunogens but does not confer immunogenicity on haptens.
Now a day’s aluminum potassium sulfate (alum) is a common adjuvant used for human vaccines. Alum prolongs the persistence of antigen. When an antigen is mixed with alum, the salt precipitates the antigen. Another commonly used adjuvant is Freund’s complete and incomplete adjuvant.
i. Freund’s incomplete adjuvant contains antigen in aqueous solution, oil, and an emulsifying agent such as mannide monooleate.
ii. Freund’s complete adjuvant contains heat-killed Mycobacteria as an additional ingredient.
Term Paper # 4.
Types of Antigens:
1. Exogenous and Endogenous Antigens:
i. Exogenous Antigens:
Antigens that originate outside the organism or cell are called as exogenous antigens. Exogenous antigens are internalized and degraded within the acidic endocytic compartments and subsequently pair with MHC class II molecules.
ii. Endogenous Antigens:
Endogenous antigens are those antigens that originate within the organism or cell. Endogenous antigens are degraded into peptides within the cytosol by proteasomes and assemble with MHC class I molecules in the Rough Endoplasmic Reticulum (RER).
2. T-Independent and T-Dependent Antigens:
i. T-Independent Antigens:
T-independent antigens are antigens which can directly stimulate the B cells to produce antibody without the requirement for T cell help. For example, polysaccharides are T-independent antigens. The responses to these antigens differ from the responses to other antigens.
ii. T-Dependent Antigens:
T-dependent antigens are those that do not directly stimulate the production of antibody without the help of T cells. Proteins are T-dependent antigens.
3. Super-Antigens:
Super-antigens are viral or bacterial proteins that bind simultaneously to the variable domain of P of a T-cell receptor and to the chain of a class II MHC molecule. Because of their unique binding ability super-antigens can activate large numbers of T cells irrespective of their antigenic specificity. Super-antigens can be exogenous and endogenous. Exogenous super-antigens are soluble proteins secreted by bacteria whereas endogenous super-antigens are cell-membrane proteins encoded by certain viruses that infect mammalian cells.