Antigen definition in immunology
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Antigen Definition in Immunology: Key Concepts and Terminology
What Is an Antigen? – Immunology Keywords
In immunology, an antigen is any substance that can be specifically recognized by the immune system, particularly by antibodies or immune cells such as B and T lymphocytes. Antigens are usually foreign to the body, including parts of pathogens like bacteria, viruses, fungi, or parasites, but can also be non-microbial substances such as pollen, drugs, or even altered host molecules. The immune system distinguishes between "self" antigens, which are usually tolerated, and "non-self" antigens, which are targeted for immune attack 13456810.
Antigen vs. Immunogen – Immunology Terminology
While the terms "antigen" and "immunogen" are often used interchangeably, they have distinct meanings in immunology. An immunogen is a substance that can both induce an immune response and react with the products of that response (such as antibodies or T cell receptors). In contrast, an antigen is any substance that can react with the products of an immune response, regardless of whether it can induce that response itself. Therefore, all immunogens are antigens, but not all antigens are immunogens 123456710.
Types and Sources of Antigens – Pathogen, Self, and Non-Self
Antigens can be classified based on their origin:
- Exogenous antigens: Found outside cells, such as on pathogens in blood or tissues, or on transplanted tissues (alloantigens).
- Endogenous antigens: Found inside cells, such as viral proteins in infected cells or proteins from cancerous cells.
- Autoantigens: Self-molecules that, if recognized by the immune system, can lead to autoimmunity 348.
Antigens can be proteins, polysaccharides, lipids, or nucleic acids, and may be natural or synthetic. They can be found on the surfaces of pathogens, as well as in non-microbial substances like drugs or environmental chemicals 45810.
Antigenic Determinants (Epitopes) – Antibody and T Cell Recognition
Antibodies and T cell receptors do not recognize the entire antigen molecule, but rather specific regions called epitopes or antigenic determinants. Epitopes are small, distinct sites on the antigen that interact with the immune system. A single antigen can have multiple epitopes, each capable of being recognized by different antibodies or T cells 3410.
Haptens – Small Molecules and Antigenicity
Some small molecules, called haptens, are not immunogenic on their own but can react with antibodies. When haptens are attached to larger carrier molecules, they can become immunogenic and stimulate an immune response. This concept is important in understanding drug allergies and vaccine design 234510.
Antigenicity vs. Immunogenicity – Key Immunology Properties
- Antigenicity: The ability of a substance to specifically bind with the products of an immune response (antibodies or T cell receptors).
- Immunogenicity: The ability of a substance to induce an immune response in the first place. Immunogenicity depends on both the nature of the antigen and the genetic and regulatory context of the host 12456.
Conclusion
In summary, an antigen in immunology is any substance that can be specifically recognized by the immune system, particularly by antibodies or immune cells. While all immunogens are antigens, not all antigens are immunogens. Antigens can be proteins, polysaccharides, lipids, or nucleic acids, and may originate from pathogens, the environment, or even the host itself. The immune system recognizes specific regions on antigens called epitopes, and small molecules called haptens can become immunogenic when attached to larger carriers. Understanding these definitions is fundamental to immunology and essential for interpreting immune responses and designing vaccines or immunotherapies 12345678+2 MORE.
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