Antigen: Definition, Types, and Examples

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Antigen Definition

Any of the many substances that, when identified by the immune system as non-self, cause an immunological response. Allergens, blood type antigens, HLA, chemicals on the surface of foreign cells, and poisons are only a few examples. “Of, relating to, or related to an antigen,” states antigenic.

What is Antigen?

An antigen is any substance that causes an immunological reaction, according to a basic definition. This reaction is designed to defend the body against dangerous things like disease-causing bacteria and cancerous development.

Antibody production, induction of cell-mediated immunity, complement activation, and the development of immunological tolerance are some of the immune responses triggered by antigens in humans. Antigens that do not trigger antibody production or cellular immunity are rare.

Immunogens are substances that may activate B or T cells and can trigger and serve as the target of an immune response on their own.

Antigen Structure

Polysaccharides or protein molecules are the most common antigens. Only when nucleic acids and lipids are joined with a polysaccharide or protein molecule do they become antigenic.

When an antibody is made, it exclusively interacts with the hapten and not with the carrier molecule. Penicillin is a hapten since the medicine is not antigenic in and of itself. However, in certain people, penicillin interacts with a particular protein in the host, triggering an immunological response.

Immunogens typically have a molecular weight of 10,000 or more. Haptens are molecules with a low molecular weight that are generally antigenic when linked to a carrier molecule.

Types of Antigen

Antigens are classified into three categories based on their origin:

1. Exogenous antigens are antigens which are not self, i.e., they are not a cellular component of the body but have managed to gain entry. They entered the body of the organism from the outside, for example, by inhalation or ingestion. Antigen-presenting cells may identify them and take them up (e.g. macrophages, dendritic cells). Allergens, proteins from donated tissues and organs, compounds on the surface of foreign cells, poisons, and other foreign particles are examples of exogenous antigens.

2. Endogenous antigens are antigens that are generated within a normal cell and then displayed on the cell surface outside of the cell. Antigens produced by infected cells, blood group antigens on the cell surface of erythrocytes (e.g., H antigen on RBCs, A antigens, and B antigens), and HLA or histocompatibility leukocyte antigens are only a few examples.

3. Autoantigens are a sort of endogenous antigen that is unique. They are wrongly recognised as nonself by immune cells, prompting the immune system to respond. These antigens are usually produced by the body’s cells. However, certain variables (for example, environmental or hereditary) prompted the immune cell to mistake them for aliens and attack them. This is how autoimmune illnesses work. Nucleoproteins and nucleic acids are examples of autoantigens.

Antigens can also be categorised depending on how they affect the immune system. An immunogen (usually a protein or polysaccharide) is an antigen that may elicit an immune response on its own, which means it can cause a lymphocyte to respond against it.

A hapten, on the other hand, is an antigen that does not trigger an immune response on its own. Before it can activate an immune response, it needs another material — a carrier molecule like albumin or globulin, for example. A hapten has a smaller molecular weight than an immunogen (which has a molecular weight of greater than 10,000). (less than 10,000).

Tolerogen is an antigen that inhibits the immunological response rather than generating it, resulting in immune tolerance. It suppresses lymphocytes by binding to their antigen receptors.

Antigen Examples

Antigens come in a variety of forms, as previously stated. Exogenous antigens, endogenous antigens, and autoantigens are three types of antigens that may be categorised based on their sources.

Exogenous antigens are digested and displayed on MHC class II, whereas endogenous antigens are presented on MHC class I, resulting in a variety of immunological responses.

Microorganisms, donated organs or tissues, and allergies can all include exogenous antigens. They’re classified as MHC class II. An endogenous antigen is one that is generated within a cell and is most normally associated as a result of viral or parasite infection. On changed cells, such as tumour cells or infected cells, endogenous cells are also present.

CTL recognise the endogenous antigen on MHC class I, allowing them to target infected or changed cells by releasing perforin, which causes holes to form with in cell membrane, affecting the cell to shrink.

An autoantigen is a natural component of the cell that is recognised by the immune system, resulting in an autoimmune disease. Natural self-antigens do not normally elicit immune responses; however, in autoimmune disorders, altered immune cells are triggered by natural self-antigens.

Antigen Detection

When the antigen is exposed to a B-cell receptor (BCR) in humoral immunity or a T-cell receptor (TCR) in cellular immunity, the antigen is recognised by the body. These antigen-specific cell receptors are extremely rare. TCRs are in charge of detecting intracellular antigens, whereas BCRs are in charge of detecting extracellular or free antigens.

Antigens can be found in the cell wall, fimbriae, flagella, capsules, bacterial toxins, viral coatings, or on the surface of other pathogens. Pathogen-associated molecular patterns are another term for pathogenic antigens (PAMPs). When PAMPs bind to a member of the toll-like receptor family, the host recognises them (TLR).

The resultant complex acts as a warning signal for the invading organism’s host. When antigenic chemicals or cells bind to BCR, they trigger extremely particular immune responses.

In order to remove the invading antigen, they promote the production of antigen-specific antibodies. T-dependent antigens require the assistance of T-helper cells in order to generate antibodies against them. Epitopes (also known as antigenic determinants) are antigen components that bind with antibodies to pathogenic cells.

At their two identical antigen-binding regions, the Y-shaped antibodies detect and adhere to each particular antigen. Human antibodies are bivalent because they only have two antigen-binding sites.

Non-microbial antigens include those present on the surface of transplanted organs or tissues, the surface of blood cells, pollen, serum proteins, and egg white. These non-microbial antigens aren’t linked to pathogens, but they can activate the adaptive immune system.

Antigens found on the surface of human red blood cells are used to classify blood types, with certain blood groups having antigens on the red blood cell surface and others lacking specific antigens.

Under normal circumstances, antigens on red blood cells are non-immunogenic. Rather, they take part in chemical reactions, aid in the attachment of other molecules, aid in the transportation of various molecules into and out of the cell, and keep the cell structure in place.

If a human receives a blood transfusion from a donor with a different blood group antigen, for example, the immune system will respond. The body identifies the transfused blood as alien cells and mounts an immunological reaction in response.

Antibodies target transfused blood antigens, which can have significant health consequences; thus, the recipient’s and donor’s blood groups should be matched before blood transfusion or organ donation to prevent this immune reaction from being triggered.

Antigen Processing and Presentation

Self-antigens are a normal part of the human body. Apoptosis eliminates B and T cells that cannot identify self-antigens, but normal B and T cells ignore self-antigens as a way of autoimmune suppression. Damage, infection, or defective apoptosis, on the other hand, can induce an autoimmune illness by triggering an immune response to self-antigens.

Intracellular pathogenic antigens that infect cells do not come into touch with circulating antibodies and hence do not have their antigens exposed. Intracellular immunity can aid in the eradication of intracellular infections as well as aberrant cells like cancer cells.

T-cells are responsible for the confronting of intracellular antigens, and they are particularly prevalent in the lungs and gastrointestinal system, where the majority of intracellular antigens enter the body.

M cells on the intestinal wall may also absorb gastrointestinal antigens, which they then transmit to antigen-presenting cells or lymphocytes to be identified by antibodies. Antigen-presenting cells ingest antigens and deliver them to other immune cells for them to confront.

Major histocompatibility complex (MHC) class I and MHC class II, which are located on the surface of antigen-presenting cells, mediate the antigen processing and presentation process. Antigen-presenting cells include B-cells, dendritic cells, and macrophages.

Dendritic cells and macrophages move to lymph nodes after ingesting antigens to deliver antigens to T-cells in lymph nodes. T-cells that are specific for this antigen then move to the antigen’s location to interact with it.

Antigen-Antibody Complex Formation

Antibody attachment to the antigen is not damaging to the antigen in any way. Other immunological processes, including agglutination, complement activation, neutralization, opsonization, and antibody-dependent cell-mediated cytotoxicity, are used to destroy pathogens.

Agglutination occurs when antibodies bind to a large number of antigens, causing them to cluster together. As a result, the number of infectious organisms that may be phagocytozed is reduced. Complement activation is a process in which antibodies stimulate the complement system to activate, resulting in inflammation and microbial lysis.

Antibodies surround the virus during neutralisation, preventing it from attaching to the host cell. In a similar way, they can neutralise bacterial poisons. The antigen-antibody complex or complement proteins can aid pathogen absorption by phagocytic cells during opsonization.

Antibody-dependent cell-mediated cytotoxicity is similar to opsonization in that antibodies cover the antigen. However, in antibody-dependent cell-mediated cytotoxicity, natural killer cells or eosinophils kill the target cell without eating it. Antigenic variation is a method by which infections might escape the immune response to antigens by changing their antigens.

Antigen and Diagnostic Immuniology

Antibodies are sufficiently particular to each antigen that they can distinguish between two isomers of amino acids in the nucleic acid sequence of the antigen. As a result, monoclonal antibodies are employed to determine and discriminate between various types of antigens by forming antigen-antibody complexes.

Precipitation reactions can be used to identify soluble antigens. The antigen-antibody complex clumps together to create a lattice, indicating the presence of a particular antigen.

In a process known as agglutination, insoluble antigens or soluble antigens that attach to a particle can be aggregated together to create different aggregates. Antibodies may counteract the damaging effects of bacterial exotoxins and prevent some viruses from adhering to host cells, therefore this response is employed to detect the presence of these viruses.

The virus gets mixed into the red blood cell-rich serum of the patient. If no hemagglutination was seen, this indicates that neutralising antibodies are present, preventing the virus from acting.

Fluorescent antibody methods are used to identify the presence of a specific antigen or antibodies that have previously been tagged and bound to antigens. When fluorescent-dye-labeled antibodies are subjected to ultraviolet light and examined using a fluorescence microscope, they glow.

The Enzyme-Linked Immunosorbent Assay (ELISA) is the most widely used immunological test for determining the presence of a specific antigen or antibodies against a specific antigen. The public uses ELISA as a pregnancy test since it is available as a prepackaged kit and the detection procedure is typically automated.

Antigen Citations
  • Design and development of therapies using chimeric antigen receptor-expressing T cells. Immunol Rev . 2014 Jan;257(1):107-26.
  • T cell antigen receptor recognition of antigen-presenting molecules. Annu Rev Immunol . 2015;33:169-200.
  • The ins and outs of MHC class II-mediated antigen processing and presentation. Nat Rev Immunol . 2015 Apr;15(4):203-16. 
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