Immunology is the branch of science that deals with the defence of the body against diseases. The immune system is efficient in protecting the body from foreign substances that disturb the normal functioning of the body. These substances against which an immune response is generated are known as antigens.
Antigens include pathogens such as bacteria, fungi, protozoa, and viruses, and also simple molecules such as proteins and polysaccharides. The antigens trigger an immune response upon binding to receptors known as antibodies, and a cascade of reactions is set into motion, ultimately leading to the elimination of the antigen. This reaction between an antigen and an antibody is very specific and dependent on a number of factors. Let us explore the nature of these antigens, their structure, and their interactions with the antibodies in a little detail.
The immune system comprising the innate and adaptive immunities ensures the healthy functioning of the human body. But, how does the body defend itself from infection? How does the body prevent the recurrence of diseases? What causes autoimmune diseases? How does vaccination prevent diseases? Immunology provides answers to a plethora of such questions. Immunology is the branch of science that deals with the components and functions of the immune system. In other words, it is the study of the body’s defense against foreign invaders. It is divided into different branches, each of which deals with a particular aspect of disease development, resistance, and treatment.
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Antigens (from antibody generators) are often defined as foreign substances that trigger an immune response in the body. Technically, however, an antigen is any substance that specifically binds to an antibody (membrane-bound or soluble) or a T-cell receptor. These agents, after binding to the specific receptors of the immune system, cause the proliferation of the immune cells through clonal selection and lead to the production of various proteins such as the cytokines and those of the complement system. All these reactions ultimately lead to the eradication of the antigen from the host’s body. Antigens can be polysaccharides, proteins, lipids or nucleic acids. Antigens are often large molecules, having a molecular weight of 10,000 Da or more.
Antigenicity refers to the capability to combine with antigen-specific receptors. The antigenicity of a substance depends on factors such as the size of the molecule, its chemical nature and the degree of foreignness to the host.
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An antigen can be comprised of proteins, polysaccharides, and sometimes even lipids and nucleic acids. An antigen bears on its surface antigenic determinant regions, known as epitopes, which bind specifically to an antibody.
Based on the dependence on T-Cells
T-dependent antigens− the antigens that depend on the helper-T cells for stimulation of antibody production by the B-cells.
Examples− proteins, polypeptides, haptens, etc.
T-independent antigens− antigens that don't require helper-T cells for stimulation of B-cells. They are less complex than T-dependent antigens.
Based on their origin
Exogenous antigens− these are foreign antigens that enter the body from external sources, via inhalation, ingestion, injection, etc. Examples include pathogenic bacteria.
Endogenous antigens− these are bacterial or viral antigens that originate from within the body, as in the case of viral proteins.
Autoantigens− these antigens are endogenous antigens that are the body’s own cells or proteins that trigger an immune response against themselves. These antigens trigger autoimmune diseases.
Based on immunogenicity
Complete antigens− these antigens can generate an immune response on their own, and are usually larger.
Haptens− These are low molecular weight substances that are non-immunogenic on their own, but can be made immunogenic by conjugation to a carrier molecule. Example− Penicillin, dinitrophenol, aminobenzene, etc.
All antigens have certain unique properties that determine their antigenicity, and these properties trigger different types of immune reactions from the host.
Most of the antigens that trigger an immune reaction are foreign substances that enter the body through different pathways.
Chemical composition of the antigen− Antigens can be of varied chemical compositions, including polysaccharides, proteins, lipids, nucleic acids, etc.
Size of the antigen − Most antigens have a molecular weight of 10,000 Da or more.
Antigenic determinants − Antigens possess unique structures on their surfaces, known as epitopes. The epitopes are involved in the interaction of the antigen with the antibody.
The valence of the antigen − The number of epitopes present on the surface of the antigen determines its valence. If only one epitope is present, the antigen is said to be monovalent. If more than one epitope is present the antigen is said to be multivalent.
Antigen-antibody interactions are analogous to enzyme-substrate reactions, in the way that they are very specific and fit into each other like a lock and key. These interactions are non-covalent and reversible. An effective defence relies on the nature of the antigen and its reaction to the antibody. Several factors affect the antigen-antibody interactions, such as −.
Degree of foreignness − The greater the degree of foreignness of the antigen, the greater the immune response of the host.
Epitope-paratope reactions − Antigen-antibody interactions are very specific, dependent on the chemical structure and the reactivity between the antigenic epitope and the part of the antibody that binds to the antigen, i.e., the paratope.
Chemical nature of the antigen − The more chemically complex the substance, the greater its immunogenicity. Hence, heteropolymers are more immunogenic than homopolymers.
The molecular size of the antigen − Generally, antigens of size smaller than 10,000 Da are said to be poor antigens, and need to be conjugated with larger molecules to enhance their immunogenicity.
The valence of the antigen and the antibody− The greater the valence of the receptor and the ligand, (i.e., antibody and the antigen), the greater the immune response. Multiple receptor-ligand interactions increase the strength of binding between the two surfaces.
Affinity − Affinity is defined as the binding strength between the antibody and the antigen. It depends on the stereochemical similarities between the two interacting molecules. The higher the affinity of an antibody for an antigen, the greater will be the stability of the reaction between them.
Avidity − Avidity refers to the overall binding strength between multivalent antibodies and antigens. Even if the affinity per binding site is low, the overall avidity of the multiple concurrent receptor-ligand interactions between an antigen and an antibody can compensate for the strength of binding between the two surfaces.
The physical form of the antigen − In general, particulate antigens trigger a greater response.
The spatial arrangement of epitopes on the antigen − When the epitopes are well separated (i.e., non-overlapping), a greater number of antibodies can bind to the antigen. However, if the epitopes are very close to each other, the binding of too many antibodies can cause steric hindrance and lead to lesser stability of the antigen-antibody complex.
Immunology is the branch of science that deals with the study of the body’s defense against diseases.
Antigens are substances that can specifically bind to an antibody.
Immunogenicity of the antigen is its ability to generate an effective immune response.
Antigen-antibody interactions depend on a number of factors including the chemical nature, the size, the valency and determinant of the antigen, and the affinity and avidity of the antigen and antibody interaction.
Antigens may be of different types depending on their origin and immunogenicity.
Q1. What are adjuvants?
Ans. Adjuvants are nonantigenic, nonimmunogenic immunopotentiators that, when added to a vaccine, enhance the immunogenicity of the vaccine and generate an effective immune response against the vaccine. Examples include− Freund’s complete adjuvant, Alum, etc.
Q2. What factors influence the avidity of an antigen-antibody interaction?
Ans. The affinity and the valency of the reacting antibody and antigens determine the overall avidity of the interaction. The greater the affinity and the valency, the stronger will the rection between the two.
Q3. What is cross-reactivity?
Ans. Cross-reactivity occurs when an antibody or a T-cell receptor, specific for a particular antigen, react with more than one antigen that possesses the same or similar epitope.
Q4. What are some clinical applications of the antigen-antibody interactions?
Ans. Antigen-antibody interactions are widely used to detect antigens in the laboratory. For example, agglutination tests are widely used in the laboratory, for blood tests. ELISA tests, which are used to detect bacterial pathogens, viruses, etc are also based on the formation of the immune complexes.
Q5. What is the difference between antigenicity and immunogenicity?
Ans. Antigenicity refers to the ability of a substance to bind to an antibody. Immunogenicity refers to the ability of an antigen to induce an immune response, involving the activation of B-cells. Antigenicity does not guarantee an effective immune response. Hence, not all antigens are immunogens but all immunogens are antigens. For example, haptens are antigens but can not generate an effective immune response without conjugation to a carrier.
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