Antigen

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Antigen

An antigen is a substance that prompts the generation of antibodies and can cause an immune response. The word originated from the notion that they can stimulate antibody generation. We now know that the immune system does not consist of only antibodies. The modern definition encompasses all substances that can be recognized by the adaptive immune system

Adaptive immune system

The adaptive immune system is composed of highly specialized, systemic cells and processes that eliminate or prevent pathogenic challenges. Thought to have arisen in the first Gnathostomata, the adaptive or "specific" immune system is activated by the ?non-specific? and evolutionarily older innate immune system ....
. In the strict sense, immunogens are those substances that elicit a response from the immune system, whereas antigens are defined as substances that bind to specific antibodies.

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Adaptive immune system

Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
s or polysaccharide

Polysaccharide

Polysaccharides are relatively complex carbohydrates. They are polymers made up of many monosaccharides joined together by glycosidic bonds. They are therefore very large, often branched, macromolecules....
s. This includes parts (coats, capsules, cell walls, flagella, fimbrae, and toxins) of bacteria

Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
, virus

Virus

A virus is a Optical microscope#Limitations of light microscopes infectious agent that is unable to grow or reproduce outside a host cell . Viruses infect all cellular life....
es, and other microorganism

Microorganism

A microorganism or microbe is an organism that is microscopic . The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design....
s. Lipid

Lipid

Lipids are broadly defined as any fat-soluble , naturally-occurring molecule, such as fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins , monoglycerides, diglycerides, phospholipids, and others....
s and nucleic acid

Nucleic acid

A nucleic acid is a macromolecule composed of chains of monomeric nucleotides. In biochemistry these molecules carry genetic information or form structures within Cell ....
s are antigenic only when combined with proteins and polysaccharides. Non-microbial exogenous (non-self) antigens can include pollen, egg white, and proteins from transplanted tissues and organs or on the surface of transfused blood cells.

Tolerogen - A substance 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. The (detrimental) reaction may result after exposure via ingestion, inhalation, injection, or contact with skin.

Cells present their antigens to the immune system via a histocompatibility molecule. Depending on the antigen presented and the type of the histocompatibility molecule, several types of immune cells can become activated.

Origin of antigens

Antigens can be classified in order of their origins.

Exogenous antigens

Exogenous antigens are antigens that have entered the body from the outside, for example by inhalation

Inhalation

Inhalation is the movement of air from the external environment, through the air ways, and into the alveoli.Inhalation begins with the onset of contraction of the diaphragm , which results in expansion of the intrapleural space and an increase in negative pressure according to Boyle's Law....
, ingestion

Ingestion

Ingestion is the consumption of a substance by an organism. In animals, it normally is accomplished by taking in the substance through the mouth into the gastrointestinal tract, such as through eating or drinking....
, or injection

Injection

Injection may refer to:* Injection , a method of putting liquid into the body with a syringe and a hollow needle that punctures the skin.* Injective function in mathematics, a function which associates distinct arguments to distinct values...
. By endocytosis

Endocytosis

Endocytosis is the process by which cell s absorb material from outside the cell by engulfing it with their cell membrane. It is used by all cells of the body because most substances important to them are large Chemical polarity molecules that cannot pass through the hydrophobic plasma membrane or cell membrane....
or phagocytosis

Phagocytosis

File:Phagocytosis in three steps.pngPhagocytosis is the cell process of Phagocytes and Protists of engulfing solid particles by the cell membrane to form an internal phagosome, which is a food vacuole, or pteroid....
, these antigens are taken into the antigen-presenting cell

Antigen-presenting cell

An antigen-presenting cell or accessory cell is a Cell that displays foreign antigen complexed with Major histocompatibility complex on its surface....
s (APCs) and processed into fragments. APCs then present the fragments to T helper cells (CD4

CD4

CD4 is a glycoprotein expressed on the surface of T helper cells, regulatory T cells, monocytes, macrophages, and dendritic cells. It was discovered in the late 1970s and was originally known as leu-3 and T4 before being named CD4 in 1984....
⁺) by the use of class II histocompatibility

MHC class II

MHC Class II molecules are found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells ....
molecules on their surface. Some T cells are specific for the peptide:MHC complex. They become activated and start to secrete cytokine

Cytokine

Cytokines are a category of signaling molecules that, like hormones and neurotransmitters, are used extensively in cell communication. They are proteins, peptides or glycoproteins....
s. Cytokines are substances that can activate cytotoxic T lymphocytes (CTL), antibody-secreting B cells, macrophages, and other particles.

Endogenous antigens

Endogenous antigens are antigens that have been generated within the cell, as a result of normal cell metabolism

Metabolism

Metabolism is the set of chemical reactions that occur in living organisms in order to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments....
, or because of viral or intracellular bacterial infection

Infection

An infection is the detrimental colonization of a host organism by a foreign species. In an infection, the infecting organism seeks to utilize the host resources to multiply ....
. The fragments are then presented on the cell surface in the complex with MHC class I

MHC class I

There are two primary classes of major histocompatibility complex molecules, class I and MHC class II. MHC class I molecules are found on almost every nucleated cell of the body....
molecules. If activated cytotoxic CD8⁺ T cells

Cytotoxic T cell

A cytotoxic T cell belongs to a sub-group of T cells that are capable of inducing the death of infection somatic or tumor cells; they kill cells that are infected with viruses , or are otherwise damaged or dysfunctional....
recognize them, the T cells begin to secrete various toxin

Toxin

A toxin is a poisonous substance produced by living cells or organisms. For a toxic substance not produced by living organisms, "toxicant" is the more appropriate term, and "toxics" is an acceptable plural....
s that cause the lysis

Lysis

Lysis refers to the death of a cell by breaking of the cellular membrane, often by viral or osmotic mechanisms that compromise its integrity. A solution containing the contents of lysed cells is called a "lysate"....
or apoptosis

Apoptosis

Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Programmed Cell death involves a series of biochemical events leading to a characteristic cell Morphology and death, in more specific terms, a series of biochemical events that lead to a variety of morphological changes, including Bleb , changes...
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 tolerance (also known as negative selection

Negative selection

Negative selection may refer to:*Negative selection , in natural selection it refers to the selective removal of rare alleles that are deleterious....
). They include xenogenic (heterologous), autologous

Autologous

In biology, autologous refers to cell , tissues or even proteins that are reimplanted in the same individual as they come from. Bone marrow, skin biopsy, cartilage, and bone can be used as autografts....
and idiotypic or allogenic (homologous) antigens. antigens are also generated between normal cells

Autoantigens

An autoantigen is usually a normal protein or complex of proteins (and sometimes DNA or RNA) that is recognized by the immune system of patients suffering from a specific autoimmune disease

Autoimmune disease

Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. In other words, the body attacks its own cells....
. These antigens should, under normal conditions, not be the target of the immune system, but, due to mainly genetic and environmental factors, the normal immunological tolerance for such an antigen has been lost in these patients.

Tumor antigens

Tumor antigens or Neoantigens are those antigens that are presented by MHC I or MHC II molecules on the surface of tumor cells. These antigens can sometimes be presented by tumor cells and never by the normal ones. In this case, they are called tumor-specific antigens (TSAs) and, in general, result from a tumor-specific mutation. More common are antigens that are presented by tumor cells and normal cells, and they are called tumor-associated antigens (TAAs). Cytotoxic T lymphocytes that recognize these antigens may be able to destroy the tumor cells before they proliferate or metastasize.

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.

Nativity

A native antigen is an antigen that is not yet processed by an APC to smaller parts. T cells cannot bind native antigens, but require that they be processed by APCs, whereas B cells can be activated by native ones.

Origin of the term antigen

In 1899 Ladislas Deutsch (Detre) (1874-1939) named the hypothetical substances halfway between bacterial constituents and antibodies "substances immunogenes ou antigenes". He originally believed those substances to be precursors of antibodies, just like zymogen is a precursor of zymase. But by 1903 he understood that an antigen induces the production of immune bodies (antibodies) and wrote that the word antigen was a contraction of "Antisomatogen = Immunkorperbildner". The Oxford English Dictionary indicates that the logical construction should be "anti(body)-gen".

Antigen

Origin of antigens

Exogenous antigens

Endogenous antigens

Autoantigens

Tumor antigens

Nativity

Origin of the term antigen

See also

External links