Antigen
In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor.[1] The presence of antigens in the body may trigger an immune response.[2]
Antigens can be proteins, peptides (amino acid chains), polysaccharides (chains of simple sugars), lipids, or nucleic acids.[3][4] Antigens exist on normal cells, cancer cells, parasites, viruses, fungi, and bacteria.[1][3]
Antigens are recognized by antigen receptors, including antibodies and T-cell receptors.[3] Diverse antigen receptors are made by cells of the immune system so that each cell has a specificity for a single antigen.[3] Upon exposure to an antigen, only the lymphocytes that recognize that antigen are activated and expanded, a process known as clonal selection.[4] In most cases, antibodies are antigen-specific, meaning that an antibody can only react to and bind one specific antigen; in some instances, however, antibodies may cross-react to bind more than one antigen. The reaction between an antigen and an antibody is called the antigen-antibody reaction.
Antigen can originate either from within the body ("self-protein" or "self antigens") or from the external environment ("non-self").[2] The immune system identifies and attacks "non-self" external antigens. Antibodies usually do not react with self-antigens due to negative selection of T cells in the thymus and B cells in the bone marrow.[5] The diseases in which antibodies react with self antigens and damage the body's own cells are called autoimmune diseases.[6]
Vaccines are examples of antigens in an immunogenic form, which are intentionally administered to a recipient to induce the memory function of the adaptive immune system towards antigens of the pathogen invading that recipient. The vaccine for seasonal influenza is a common example.[7]
Etymology[edit]
Paul Ehrlich coined the term antibody (German: Antikörper) in his side-chain theory at the end of the 19th century.[8] In 1899, Ladislas Deutsch (László Detre) named the hypothetical substances halfway between bacterial constituents and antibodies "antigenic or immunogenic substances" (French: substances immunogènes ou antigènes). He originally believed those substances to be precursors of antibodies, just as a zymogen is a precursor of an enzyme. But, by 1903, he understood that an antigen induces the production of immune bodies (antibodies) and wrote that the word antigen is a contraction of antisomatogen (Immunkörperbildner). The Oxford English Dictionary indicates that the logical construction should be "anti(body)-gen".[9] The term originally referred to a substance that acts as an antibody generator.[10]
Antigen-presenting cells present antigens in the form of peptides on histocompatibility molecules. The T cells selectively recognize the antigens; depending on the antigen and the type of the histocompatibility molecule, different types of T cells will be activated. For T-cell receptor (TCR) recognition, the peptide must be processed into small fragments inside the cell and presented by a major histocompatibility complex (MHC).[12] The antigen cannot elicit the immune response without the help of an immunologic adjuvant.[13] Similarly, the adjuvant component of vaccines plays an essential role in the activation of the innate immune system.[14][15]
An immunogen is an antigen substance (or adduct) that is able to trigger a humoral (innate) or cell-mediated immune response.[16] It first initiates an innate immune response, which then causes the activation of the adaptive immune response. An antigen binds the highly variable immunoreceptor products (B-cell receptor or T-cell receptor) once these have been generated. Immunogens are those antigens, termed immunogenic, capable of inducing an immune response.[17]
At the molecular level, an antigen can be characterized by its ability to bind to an antibody's paratopes. Different antibodies have the potential to discriminate among specific epitopes present on the antigen surface. A hapten is a small molecule that can only induce an immune response when attached to a larger carrier molecule, such as a protein. Antigens can be proteins, polysaccharides, lipids, nucleic acids or other biomolecules.[4] This includes parts (coats, capsules, cell walls, flagella, fimbriae, and toxins) of bacteria, viruses, and other microorganisms. Non-microbial non-self antigens can include pollen, egg white, and proteins from transplanted tissues and organs or on the surface of transfused blood cells.
Antigenic specificity[edit]
Antigenic specificity is the ability of the host cells to recognize an antigen specifically as a unique molecular entity and distinguish it from another with exquisite precision. Antigen specificity is due primarily to the side-chain conformations of the antigen. It is measurable and need not be linear or of a rate-limited step or equation.[2][7] Both T cells and B cells are cellular components of adaptive immunity.[2][4]