Table of Contents
Introduction
Antigens are large 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.
Properties of antigens
The properties of antigens are as follows:
- The antigen should be a foreign substance to induce an immune response.
- The antigens have a molecular mass of 14,000 to 6,00,000 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 influence the immunogenicity, very young and very old people exhibit very low immunogenicity.
Types of Antigens
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Antigens are classified as two types —
- On the basis of origin
- On the basis of immune response.
1. On the basis of origin:
a. Exogenous antigen: Exogenous antigens are the external antigens that enter the body from outside. E.g. inhalation, injection,e.t.c. These include food allergen, pollen, aerosols, etc. and are the most common type of antigens.
b. Endogenous Antigens: Endogenous antigens are generated inside the body due to viral or bacterial infections or cellular metabolism.
c. Autoantigens: Autoantigens are the self proteins or nucleic acids that due to some genetic or environmental alterations get attacked by their own immune system causing autoimmune diseases.
d. Tumour Antigens: It is an antigenic substance present on the surface of tumour cells that induces an immune response in the host, e.g. MHC-I and MHC-II. Many tumours develop a mechanisms to evade the immune system of the body.
e. Negative antigens: An antigen that is not yet processed by an antigen - presenting cell is known as native antigens.
2. On the basis of immune response: Antigens are classified as —
a. Immunoge : These may be proteins or polysaccharides and can generate an immune response on their own.
b. Hapten: These are non- proteins, foreign substances that require a carrier molecule to induce an immune response.
B and T cell epitopes
B and T cell epitopes are specific regions on antigens that are recognized by B cells and T cells, respectively, playing crucial roles in the immune response.
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B cell epitopes
B cell epitopes are specific parts of an antigen that are recognized and bound by B cell receptors or antibodies. These epitopes are typically located on the surface of the antigen and are accessible to the immune system. B cells epitopes can be:
- Linear Epitopes: These are continuous sequences of amino acids in a polypeptide chain. Linear epitopes maintain their structure regardless of the protein conformation.
- Conformational Epitopes: These consists of amino acids that are brought together in the three - dimensional structure of the protein. They are often non - contiguous in the primary sequence but are spatially close in the folded protein.
T cell epitopes
T cell epitopes are specific peptide fragments presented by major Histocompatibility Complex (MHC) molecules on the surface of antigen - presenting cells (APCs). These episodes are recognised by T cell receptors (TCRs) on T cells. The process involves several steps:
- Antigen Processing: Proteins are degraded into peptide fragments within the cell.
- Antigen Presentation: Peptide fragments are loaded onto MHC molecules. MHC class I molecules present peptides to CD8+ T cells (Cytoxic T cells), while MHC class II molecules present peptides to CD4+ T cells (helper T cells)
T cell epitopes are generally linear peptides of about 8 -15 amino acids for MHC class I and 13 - 25 amino acids for MHC class II.
Hapten and adjuvants
Haptens
Haptens are small molecules that cannot trigger an immune response on their own. However, when they attach to a larger molecule, typically a protein, they form a complex that the immune system can recognize and respond to. This is important in the study of allergies, autoimmunity, and vaccine development, where haptens can help produce antibodies against small molecules. Haptens are usually low molecular weight compounds. Due to their small size, haptens cannot induce an immune response unless attached to a larger carrier molecule.
Adjuvants
Adjuvants are substances added to vaccines to boost the body's immune response to an antigen. They work by stimulating the immune system, making the vaccine more effective, and often allowing for a smaller amount of antigens to be used. Adjuvants are particularly useful when the antigen alone does not provoke a strong immune response. Common adjuvants include aluminium salts, oil - in - water emulations, and certain bacterial components. They can help to deliver antigens to antigen - presenting cells (APCs) such as dendritic cells, enhancing the presentation of the antigen to T cells.
FAQs
What are antigens?
Antigens are molecules or molecular structures that are recognized by the immune system, specifically by antibodies, B cells, or T cells. They can be proteins, polysaccharides, lipids, or nucleic acids.
What are the basic properties of antigens?
The basic properties of antigens include their ability to induce an immune response (immunogenicity) and their ability to specifically react with the products of that response (antigenicity).
What are B cell epitopes?
B cell epitopes are specific regions on an antigen that are recognized and bound by B cell receptors or antibodies. These epitopes are typically accessible on the surface of the antigen.
What are T cell epitopes?
T cell epitopes are short peptides derived from antigens that are presented on the surface of cells by Major Histocompatibility Complex (MHC) molecules. T cells recognize these epitopes via their T cell receptors (TCRs).
