What Are Antigens? -Definition, Properties, Types, Structure
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Course
BCH191
Institution
Bharathidasan University
Do You Know About Antigens? Here the Overview of Antigens. It Include Definition, Properties, Structure, Types, Processing and Presentation, Antigen- Antibody Complex, Applications.
Antigens are molecules or molecular structures that are foreign to the body and generally induce
an immune reaction in the form of the production of antibodies against them.
Antigen
• In simple words, antigens can be anything that doesn’t belong to the body and are foreign.
• Even though antigens are usually defined by the induction of an immune response, all
antigens might not induce an immune response. The antigens that induce a response are
termed immunogens.
• The ability of antigens to elicit an immune response depends on the presence of specific
regions on the antigens called antigenic determinants. The determinants bind to receptor
molecules with the complementary structure on immune cells to elicit a response.
• Antigens are indicated by the term ‘Ag’, and these can occur in different forms like pollen,
viruses, chemicals, or bacteria.
• The concept of antigen arose from the fact that our body can distinguish between the
components of the body and foreign particles.
• In response to these antigens, the body induces the production of antibodies that act
against the said antigens.
• Most antigens in humans are proteins, peptides, or polysaccharides; however, lipid and
nucleic acids can also act as antigens when combined with proteins or polysaccharides.
• In addition, antigens might also be intentionally introduced into the body in the form of
vaccines in order to induce the adaptive immune system of the body against the antigen.
Properties of Antigens
, Antigens have different properties which determine the immunogenicity of the antigens and thus
are essential in order to understand the immune reaction against them. Since these properties
determine the immunogenicity, these are considered properties required to form a good antigen.
The following are some of the properties of antigens;
1. Foreign Nature
• All antigens that induce an immune response in the host are foreign to the body of the
recipient.
• The host body recognizes the antigen to be different from the normal body components.
• The immunogenicity of the antigen increases with the increase in the degree of
foreignness. In the case of biological antigens, the foreignness increases with the increase
in the phylogenetic gap between the two species.
• However, there are some exceptions in that some proteins occurring within the host might
also induce an immune response, as in the case of autoantigens.
• Similarly, proteins and other molecules from other species might also not induce an
immune response if they lack antigenic determinants or epitopes.
2. Chemical Nature
• The most potent and commonly encountered antigens are proteins followed by
polysaccharides.
• However, other molecules like lipids and nucleic acids can also act as antigens when
complex with proteins and polysaccharides.
• In the case of proteins, the antigen should contain immunogenic regions with at least 30%
of amino acids like lysine, glutamine, arginine, glutamic acids, aspargine, and aspartic acid,
along with a high number of hydrophilic or charged groups.
• The level of Immunogenicity also increases with the heterogenicity of the molecules.
Homopolymers are usually less immunogenic than heteropolymers.
3. Molecular Size
• The molecular size of the antigens is also crucial in the immunogenicity of the molecules.
• It has been established that antigens should have a minimum size of greater than 5000 Da
before they can be considered immunogenic.
• However, low molecular weight substances can demonstrate immunogenicity when
coupled with large-sized carriers.
• The low molecular weight substances are termed haptens that are considered ‘partial
antigens’ with at least one antigenic determinant.
4. Molecular Rigidity and Complexity
• The rigidity and complexity of molecules are essential factors that determine
immunogenicity.
• In general, rigid molecules are good antigens as they can raise antibodies to certain
structures when compared to the less rigid ones.
• The complexity of the structure is also an essential factor as a peptide antigen with a
repeating unit of a single amino acid is less immunogenic than a molecule with two or more
repeating amino acids units.
5. Antigenic Determinants and Cross-reactivity
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