An antigen is a molecule that stimulates the production of antibodies. Usually, it is a protein or a polysaccharide, but can be any type of molecule, including small molecules (haptens) coupled to a protein (carrier). Wikipedia does not yet have an article with this exact name. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Polysaccharides (sometimes called glycans) are relatively complex carbohydrates. ... A hapten is a small molecule which can elicit an immune response only when attached to a large carrier such as a protein; the carrier may be one which also does not elicit an immune response by itself. ... Carrier has several different meanings: in telecommunication, a carrier wave in biology, an asymptomatic carrier or a carrier-protein the Carrier tribe, a First Nations tribe living in British Columbia, Canada; also the name of their Athabaskan language a common carrier, a transport business (shipping or telecom) an aircraft carrier...

Types of antigens

• Immunogen - Any substance that provokes the immune response when introduced into the body. An immunogen is always a macromolecule (protein, polysaccharide). Its ability to provoke the immune response depends on its foreignness to the host, molecular size, chemical composition and heterogeneity (e.g. different amino acids in a protein).
• Tolerogen - An antigen that invokes a specific immune non-responsiveness due to its molecular form. If its molecular form is changed, a tolerogen can become an immunogen.
• Allergen - An allergen is a substance that causes the allergic reaction. It can be ingested, inhaled, injected or comes into contact with skin.

Cells present their antigens to the environment via a histocompatibility molecule. Depending on the antigen presented and the type of the histocompatibility molecule, several types of immune cells can become activated. A macromolecule is a molecule composed of a very large number of atoms. ... Geometry of the water molecule Molecules have fixed equilibrium geometries--bond lengths and angles--that are dictated by the laws of quantum mechanics. ... This article needs cleanup. ... The major histocompatibility complex (MHC) is a large genomic region or gene family found in most vertebrates containing many genes with important immune system roles. ... White Blood Cells is also the name of a White Stripes album. ...

Origin of antigens

We can also classify antigens according to their origin.

Exogenous antigens

Exogenous antigens are antigens that have entered the body form the outside, for example by inhalation, ingestion, or injection. By endocytosis or phagocytosis, these antigens are taken into the antigen-presenting cells (APCs) and processed into fragments. APCs then present the fragments to T helper cells (CD4⁺) by the use of class II histocompatibility molecules on their surface. Some T cells are specific for the peptide:MHC complex. They become activated and start to secrete cytokines. Cytokines are substances that can activate cytotoxic T lymphocytes (CTL), antibody-secreting B cells, macrophages and other cells. Endocytosis is a process whereby cells absorb material (molecules or other cells) outside their cell membranes. ... In phagocytosis (literally, cell eating), large particles are enveloped by the cell membrane of a (usually larger) cell and internalized to form a food vacuole. ... An antigen presenting cell (APC) is a cell that displays foreign antigen complexed with MHC on its surface. ... T cells are a subset of lymphocytes that play a large role in the immune response. ... CD4 (cluster of differentiation 4) is a molecule that is expressed on the surface of T helper cells (as well as regulatory T cells and dendritic cells). ... The major histocompatibility complex (MHC) is a large genomic region or gene family found in most vertebrates containing many genes with important immune system roles. ... Cytokines are small protein molecules that are the core of communication between immune system cells, and even between these cells and cells belonging to other tissue types. ... A cytotoxic (or TC) T cell is a T cell (a type of white blood cell) which has on its surface antigen receptors that can bind to fragments of antigens displayed by the Class I MHC molecules of virus infected somatic cells and tumor cells. ... B cells are lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response). ... Macrophages (Greek: big eaters) are cells found in tissues that are responsible for phagocytosis of pathogens, dead cells and cellular debris. ...

Endogenous antigens

Endogenous antigens are antigens that have been generated within the cell, as a result of normal cell metabolism, or because of viral or intracellular bacterial infection. The fragments are then presented on the cell surface in the complex with class I histocompatibility molecules. If an activated cytotoxic CD8⁺ T cells recognize them, the T cells begin to secrete different toxins that cause the lysis or apoptosis of the infected cell. In order to keep the cytotoxic cells from killing cells just for presenting self-proteins, self-reactive T cells are deleted from the repertoire as a result of central tolerance (also known as negative selection which occurs in the thymus). Only those CTL that do not react to self-peptides that are presented in the thymus in the context of MHC class I molecules are allowed to enter the bloodstream. Santorio Santorio (1561-1636) in his steelyard balance, from Ars de statica medecina, first published 1614 Metabolism (from μεταβολισμος(metavallo), the Greek word for change), in the most general sense, is the ingestion and breakdown of complex compounds, coupled with the liberation of energy, and the consequent generation of waste... An infection is the detrimental colonization of a host organism by a foreign species. ... The major histocompatibility complex (MHC) is a large genomic region or gene family found in most vertebrates containing many genes with important immune system roles. ... T cells are a subset of lymphocytes that play a large role in the immune response. ... A toxin, in a scientific context, is a biologically produced substance that causes injury to the health of a living thing on contact or absorption, typically by interacting with biological macromolecules such as enzymes and receptors. ... Lysis (Greek lusis from luein = to separate) is the reduction of symptoms of a disease the dissolving of cells osmotic lysis chemical lysis viral lysis a dialogue of Plato about friendship (philia) This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same... In biology, apoptosis (from the Greek words apo = from and ptosis = falling, pronounced ap-a-tow-sis[1]) is one of the main types of programmed cell death (PCD). ... In biology, negative selection is artificial selection in which negative, rather than positive traits of a species are selected. ... Thymus, see Thyme. ... MHC class I molecules are cell surface proteins found on most cells of the body. ...

There is an exception to the exogenous/endogenous antigen paradigm, called cross-presentation.

Tumor antigens

Tumor antigens are those antigens that are presented by the MHC I molecules on the surface of tumor cells. These antigens can sometimes be presented only by tumor cells and never by the normal ones. In this case, they are called tumor-specific antigens and typically result from a tumor specific mutation. More common are antigens that are presented by tumor cell sand normal cells, and they are called tumor-associated antigens. Cytotoxic T lymphocytes that recognized these antigens may be able to destroy the tumor cells before they proliferate or metastasize. MHC class I molecules are cell surface proteins found on most cells of the body. ... See the article about cancer for the main article about malignant tumors. ... A cytotoxic (or TC) T cell is a T cell (a type of white blood cell) which has on its surface antigen receptors that can bind to fragments of antigens displayed by the Class I MHC molecules of virus infected somatic cells and tumor cells. ... Metastasis (Greek: change of the state) is the spread of cancer from its primary site to other places in the body. ...

Tumor antigens can also be on the surface of the tumor in the form of, for example, a mutated receptor, in which case they will be recognized by B cells. B cells are lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response). ...