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An antibody is a Y-shaped protein that is produced by B cells to identify and neutralize antigens in the body.
Describe antibody structure and function
An antibody, also known as an immunoglobulin, is a large Y-shaped protein produced by B-cells that is used by the immune system to identify and neutralize foreign objects such as bacteria and viruses.
Each tip of the "Y" of an antibody contains a paratope (a structure analogous to a lock) that is specific for one particular epitope (similarly analogous to a key) on an antigen, allowing these two structures to bind together with precision.
Though the general structure of all antibodies is very similar, a small region at the tip of the protein is extremely variable, allowing millions of antibodies with different antigen binding sites, to exist. This region is known as the hypervariable region.
There are 5 isotypes of antibodies that are found in different locations and perform different specific functions.
The base of the Y plays a role in modulating immune cell activity. This region is called the Fc region, and phagocytes may bind to it to initiate phagocytosis.
Antibodies that bind to surface antigens on, for example, a bacterium attract the first component of the complement cascade with their Fc region and initiate activation of the "classical" complement system.
An antibody (formally called immunoglobulin) is a large Y-shaped glycoprotein produced by B-cells and used by the immune system to identify and neutralize pathogens. Antibodies are produced by B cells, and are either secreted into circulation, or remain expressed on the surface of the B cell.
The antibody recognizes a unique part of an antigen (foreign object). Each tip of the "Y" of an antibody contains a paratope (a structure analogous to a lock) that is specific for one particular epitope (similarly analogous to a key) on an antigen, allowing these two structures to bind together with precision. Using this binding mechanism, an antibody can neutralize its target directly, or tag it for attack by other parts of the immune system.
Antibodies are glycoproteins belonging to the immunoglobulin superfamily, and are typically made of basic structural units—each with two large heavy chains and two small light chains. Most antibodies exist as a monomer, in which they have a single "Y" shaped sub-unit, but some antibodies can exist as dimers (two subunits) or pentamers (five subunits). The paratope is considered to be a hypervariable region, and has the same specificity and antigen binding affinity as the B cell receptor of the B cell that created the antibody. In some isotypes, the tail end of the antibody is called the constant region, which faces away from the "Y-shaped" paratobe ends, functions as an Fc tail that phagocytes can bind to.
There are five different isotypes of antibodies. They all perform different functions and found are generally found in different parts of the body.
IgA: A dimer that is secreted into mucosal surfaces, such as the gut, respiratory tract, and urogenital tract, and prevents mucosal invasion into the body by pathogens. It is resistant to the proteolytic enzymes found in the gastrointestinal mucosae.
IgD: Functions mainly as an antigen receptor on B cells that have not been exposed to antigens. It has been shown to activate basophils and mast cells to produce antimicrobial factors.
IgE: Found in circulation, and binds to allergens and triggers histamine release from mast cells and basophils, and is involved in allergy. Also protects against parasitic helminthes worms.
IgG: Has four different forms, and provides the majority of antibody-based immunity against invading pathogens, because it is the best opsonin of any type of antibody. This is because it expresses a tail for Fc receptors on phagocytes to bind to, which activates phagocytosis. It is the only antibody capable of crossing the placenta to give passive immunity to fetus, and can activate the classical complement system.
IgM: Expressed on the surface of B cells (monomer) and in a secreted pentamer with very high avidity. Eliminates pathogens in the early stages of B cell mediated (humoral) immunity before there is sufficient IgG. Like IgG, it can also activate the classical complement system.
Function of Antibodies
Circulating antibodies are produced by clonal B cells that specifically respond to only one antigen. Antibodies contribute to immunity in three ways: preventing pathogens from entering or damaging cells by binding to them (neutralization); stimulating removal of pathogens by macrophages and other cells by coating the pathogen (opsonization); and triggering destruction of pathogens by stimulating other immune responses such as the complement pathway. The complement system starts a long cascade of protein productions that either opsonize a pathogen for phagocytosis, or lyse it directly by forming a membrane attack complex. During opsonization, the antibody will express the tail for an Fc receptor on a macrophage, neutrophil, or natural killer cell. The immune cell will then bind to the antibody's Fc tail instead of the pathogen itself, which speed up the process of finding pathogens to phagocytize. Additionally, because antibodies have two or more paratopes, they can sometimes link pathogens together, which makes phagocytosis more efficient.
Source: Boundless. “Structure and Function of Antibodies.” Boundless Anatomy and Physiology. Boundless, 27 Jun. 2016. Retrieved 25 Sep. 2016 from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/immune-system-21/humoral-immune-response-201/structure-and-function-of-antibodies-985-10108/