Definition of antigen
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Definition of Antigen
Introduction to Antigens
An antigen is a substance that can trigger an immune response in the body. This response is typically characterized by the production of specific antibodies or the activation of immune cells that target the antigen Rahman2019Hammarström1985Cruse1999. Antigens are crucial in the body's defense mechanism against pathogens such as bacteria, viruses, fungi, and parasites .
Types of Antigens
Complete Antigens
Complete antigens, also known as immunogens, are substances that can both induce an immune response and react with the products of that response, such as antibodies or T-cell receptors Cruse1999Kabat1980. These antigens are typically large molecules, such as proteins or polysaccharides, that are recognized as foreign by the immune system Sela1996Mcduffie1981.
Incomplete Antigens (Haptens)
Incomplete antigens, or haptens, are small molecules that cannot induce an immune response on their own. However, when they are attached to a larger carrier molecule, they can become immunogenic and elicit an immune response Cruse1999Kabat1980Lu2016. Examples of haptens include small peptides, pesticides, antibiotics, and hormones .
Molecular Nature of Antigens
Antigens can be composed of various types of molecules, including proteins, polysaccharides, lipids, and nucleic acids Sela1996Mcduffie1981. The specific part of the antigen that is recognized by the immune system is called an epitope. Epitopes can be linear, consisting of a sequence of amino acids, or conformational, formed by the three-dimensional structure of the antigen .
Role in Immune Response
When an antigen enters the body, it is recognized as foreign by the immune system, which then mounts a response to eliminate it. This response can involve the production of antibodies by B cells, the activation of T cells, or both Cruse1999Kabat1980. The immune system's ability to distinguish between self and non-self is crucial in preventing autoimmune reactions, where the body attacks its own tissues .
Applications in Medicine
Antigens play a vital role in the development of vaccines and diagnostic tests. Vaccines often contain antigens that stimulate the immune system to produce a protective response without causing disease . In diagnostic tests, such as immunoassays, antigens are used to detect the presence of specific antibodies in a sample, indicating exposure to a particular pathogen .
Conclusion
In summary, antigens are substances that can trigger an immune response, leading to the production of antibodies or the activation of immune cells. They can be complete antigens, capable of inducing an immune response on their own, or incomplete antigens, which require a carrier molecule to become immunogenic. Understanding the nature and function of antigens is essential in immunology, vaccine development, and diagnostic testing.
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