Antigen and Immunology - Definition, Properties, Types, Structure, Function

An antigen is a molecule that initiates the production of an antibody to help our immune system be prepared against invaders and causes an immune response. Immunology is the study of the immune system and how it protects the body from infections, diseases, and foreign substances. Antigen and Immunology are the topics of the chapter on Human Health and Disease in Biology. This article will cover the antigen definition, types, structure and properties.

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This Story also Contains

What is an Antigen?
Antigen Structure
Properties of Antigen
Types of Antigen Based on Origin
Types of Antigen Based on Immune Response
How Antigens Work
Functions of Antigens
Antigens and the Immune System
Tips, Tricks, and Strategies for Antigens and Immunology
Recommended Video on Antigen

Antigen and Immunology: Definition, Properties, Types, Structure, Function

What is an Antigen?

An antigen is any substance that triggers an immune response by binding to specific antibodies or immune cells. Antigens are foreign molecules like proteins or polysaccharides on pathogens (such as bacteria and viruses), but they can also include non-infectious substances like pollen or toxins. Some important antigen notes are discussed below:

Antigens are recognised and bound by an antibody or immune cell receptor which is called epitome.
Vaccines contain antigens from pathogens which helps the immune system to recognise and remember the pathogen.
Antigens also stimulate the production of antibodies by B cells and activate the T-cells.
They play a very crucial role in the entire immune system.

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Antigen Structure

The structure of antigens varies depending on their origin, but most antigens are large biomolecules, such as proteins, polysaccharides, lipids, or nucleic acids. Epitopes, also known as antigenic determinants, are antigen components. Every antigen contains several epitopes.

Molecular Structure

Normally, antigens are proteins, polysaccharides, lipids, or nucleic acids. Their complex molecular architecture allows them to be recognised as non-self.

Epitope Explanation

An epitope, otherwise known as an antigenic determinant, is the portion of an antigen that binds to a particular antibody. An antigen possesses more than one epitope, which presents a diversified type of immune response.

Diagram of Antigen Structure

The diagram below shows the structure of antigen

Antigen Structure

Properties of Antigen

Antigens are substances that are so important because they help our immune system be prepared against invaders.

Property	Description
Immunogenicity	The property of an antigen by which it can stimulate an immune response.
Antigenicity	The ability of an antigen to combine specifically with the products of the immune response (for example, antibodies or T-cell receptors).
Molecular Size	Larger molecules are, in general, more immunogenic than smaller ones.
Complexity	More complex molecules, such as proteins, are more likely to be antigenic than simpler ones.
Degradability	The immunogenicity of an antigen reflects its capacity to be processed by the immune system and affects how well it can be presented to the immune cells.

Types of Antigen Based on Origin

Antigens have diverse types, depending on their sources, and each has very unique characteristics. The different types of antigen are discussed below:

Type of Antigens	Characteristics	Examples
Exogenous Antigens	These antigens enter the body from external sources.	Pollen grains, dust mites and flu virus.
Endogenous Antigens	Antigens are produced within the body's cells and result in Infection or cellular damage.	Viral protein on the infected cells, the abnormal protein on the cancer cell.
Autoantigens	Normal cellular proteins that the immune system mistakenly recognises as foreign agents.	Insulin Type 1, thyroid and thyroid receptors.
Neoantigens	Newly formed antigens due to genetic mutations. These are generally related to cancer.	Tumour specific antigens.

Types of Antigen Based on Immune Response

Antigens possess this unique quality of triggering an immune response, so they initiate the production of antibodies, which protect our body from harmful invaders. Depending on the type of immune response elicited, there are different categories of antigens.

Complete Antigen: They are also known as immunogens. It can induce an immune response. These antigens can be found in pathogen-associated molecules, such as the proteins in bacteria or viruses.
Incomplete Antigens: They are also known as haptens. The antigens are too small to carry an immune response by themselves and need to be conjugated with a carrier protein to be able to carry an immune response.

How Antigens Work

The interaction of antigens with the immune system is explained below:

Mechanism	Explanation
Interaction with the Immune System	The antigens interact with the immune system by binding to its specific receptors on the immune cells. Binding would trigger a cascade of immune responses in regards to a pathogen's neutralisation.
Role in Triggering Immune Responses	Whenever there is contact with an antigen, the immune system activates lymphocytes. B-cells produce antibodies. T-cells attack infected cells by orchestrating combined defence.
Antigen-Antibody Interaction	The competition for antigen binding is also a major feature of the humoral responses. This is where antibodies produced exclusively by B-cells bind to antigens, neutralising them with high specificity and marking them to be destroyed by other immune cells.

Diagram of Antigen and Antibody

The diagram below shows the structure of antigen and antibody and their mechanism of action

Antigen and Antibody

Functions of Antigens

Some Important function of antigens is discussed below:

The antigen is important in the induction of both innate and adaptive immune responses, protecting the host from infection.
Vaccines use inactivated or weakened antigens to stimulate the immune system with the help of immunity, not causing disease.
At times, antigens cause hypersensitivity reactions, such as allergies and autoimmune diseases, wherein the immune system reacts too much.

Applications of Antigens in the Treatment of Diseases

Antigens find wide application in research labs in studies of the immune response and construction of new treatments. Some of them are discussed below:

Antigens find application/purpose in diagnostic tests such as the ELISA test to check for the presence of antibodies against a given sample.
Antigen-based therapies include vaccines and monoclonal antibodies for the treatment of infections and cancer.
The ELISA test detects specific antigens or antibodies in a sample; it is the assay widely used for diagnosing diseases like HIV and COVID-19.

Antigens and the Immune System

Role in Innate Immunity: It is just in innate immunity that antigens are seen as a result of this recognition by PRRs on the surface of immune cells, and an immediate but non-specific response is induced.

Role in Adaptive Immunity: Adaptive immunity is the specific recognition of antigens by B-cells and T-cells, which results in a targeted and long-lasting immune response.

Tips, Tricks, and Strategies for Antigens and Immunology

It takes effort to remember everything in a single go. We made the entire problem easy. Some of the tricks regarding Antigen and Immunology are given below which you can use to memorise the important points.

Types of Antigens Based on Origin

"ENA-N: Exogenous, Endogenous, Autoantigens, Neoantigens"