An antigen (from antibody-generating) or immunogen is a substance that prompts the generation of antibodies and can cause an immune response. Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily ^[1] 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. Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily The modern definition encompasses all substances that can be recognized by the adaptive immune system. Immune system|Passive immunity|Innate immune system The adaptive immune system is composed of highly specialized systemic cells and processes that eliminate or prevent Pathogenic

Antigens are usually proteins or polysaccharides. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Polysaccharides are relatively complex Carbohydrates They are Polymers made up of many Monosaccharides joined together by Glycosidic bonds This includes parts (coats, capsules, cell walls, flagella, fimbrae, and toxins) of bacteria, viruses, and other microorganisms. The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have A virus (from the Latin virus meaning Toxin or Poison) is a sub-microscopic infectious agent that is unable A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually Lipids and nucleic acids are antigenic only when combined with proteins and polysaccharides. Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble A nucleic acid is a Macromolecule composed of chains of monomeric Nucleotides In Biochemistry these Molecules carry Genetic information 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. Molecular geometry or molecular structure is the three- Dimensional arrangement of the Atoms that constitute a Molecule. If its molecular form is changed, a tolerogen can become an immunogen.
• Allergen - An allergen is a substance that causes the allergic reaction. Allergy is a disorder of the Immune system often also referred to as Atopy. 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. The major histocompatibility complex ( MHC) is a large genomic region or Gene family found in most Vertebrates It is the most gene-dense region Depending on the antigen presented and the type of the histocompatibility molecule, several types of immune cells can become activated.

Contents 1 Origin of antigens 1.1 Exogenous antigens 1.2 Endogenous antigens 1.3 Autoantigens 2 Tumor antigens 3 Nativity 4 See also 5 References 6 External links

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, ingestion, or injection. By endocytosis or phagocytosis, these antigens are taken into the antigen-presenting cells (APCs) and processed into fragments. Endocytosis is a process where cells absorb material ( Molecules such as proteins from the outside by engulfing it with their Cell membrane. Phagocytosis is the cellular process of engulfing solid particles by the Cell membrane to form an internal Phagosome, or "food vacuole See also Antigen presentation An antigen-presenting cell ( APC) or accessory cell is a cell that displays foreign Antigen complexed APCs then present the fragments to T helper cells (CD4⁺) by the use of class II histocompatibility molecules[1] on their surface. T helper cells (also known as effector T cells or Th cells) are a sub-group of Lymphocytes (a type of White blood cell or CD4 ( Cluster of differentiation 4 is a Glycoprotein expressed on the surface of T helper cells, Regulatory T cells, Monocytes, Some T cells are specific for the peptide:MHC complex. They become activated and start to secrete cytokines. Cytokines are a category of signalling Proteins and Glycoproteins that like Hormones and Neurotransmitters, are used extensively in cellular Cytokines are substances that can activate cytotoxic T lymphocytes (CTL), antibody-secreting B cells, macrophages, and other particles. A cytotoxic T cell (also known as TC, CTL T-Killer cell cytolytic T cell CD8+ T-cells or killer T cell) belongs to a sub-group of T lymphocytes B cells are Lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response, which is governed by Macrophages ( Greek: "big eaters" from makros "large" + phagein "eat" ( Mø) are cells within the tissues that

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. Metabolism is the set of Chemical reactions that occur in living Organisms in order to maintain Life. An infection is the detrimental Colonization of a host Organism by a foreign Species. The fragments are then presented on the cell surface in the complex with MHC class I molecules. There are two primary classes of Major histocompatibility complex (MHC molecules class I and MHC class II. If activated cytotoxic CD8⁺ T cells recognize them, the T cells begin to secrete various toxins that cause the lysis or apoptosis of the infected cell. A cytotoxic T cell (also known as TC, CTL T-Killer cell cytolytic T cell CD8+ T-cells or killer T cell) belongs to a sub-group of T lymphocytes A toxin ( Greek:, toxikon, lit (poison for use on arrows is a Poisonous substance produced by living cells or organisms that is active at very low Lysis ( Greek, lysis from lyein = to separate refers to the death of a cell by breaking of the cellular membrane often by viral or osmotic 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).

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. Autoimmunity is the failure of an organism to recognize its own constituent parts as self, which results in an immune response against its own cells and tissues Autoimmune diseases arise from an overactive Immune response of the body against substances and tissues normally present in the body 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. Immune or immunological tolerance is the process by which the Immune system does not attack an Antigen.

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. There are two primary classes of Major histocompatibility complex (MHC molecules class I and MHC class II. MHC ( Major histocompatibility complex) Class II molecules are found only on a few specialized cell types including Macrophages Dendritic cells and See also Cancer A tumor or tumour is the name for a swelling or lesion formed by an abnormal growth of cells (termed neoplastic 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. A cytotoxic T cell (also known as TC, CTL T-Killer cell cytolytic T cell CD8+ T-cells or killer T cell) belongs to a sub-group of T lymphocytes Metastasis ( Greek: displacement μετά=next + στάσις=placement, plural metastases) sometimes abbreviated mets,

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, which is governed by

Nativity

A native antigen is an antigen which isn't yet processed by an APC to smaller parts. T cells cannot bind native antigens, but require that they are processed by APCs, while B cells can be activated by native ones. T cells belong to a group of White blood cells known as Lymphocytes, and play a central role in Cell-mediated immunity. B cells are Lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response, which is governed by

See also

• Epitope
• Original antigenic sin

References

External links

• Antigen Retrieval Protocol
• Immunology
• National Library of Medicine/Medline (National Insititute of Health) website

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