Immunoglobulin G
Encyclopedia
Immunoglobulin G are antibody
molecules. Each IgG is composed of four peptide
chains — two heavy chains γ and two light chains. Each IgG has two antigen
binding sites. Other immunoglobulins may be described in terms of polymers with the IgG structure considered the monomer
.
IgG constitutes 75% of serum
immunoglobulins in humans. IgG molecules are synthesized and secreted by plasma B cells.
. The presence of specific IgG, in general, corresponds to maturation of the antibody response. Human IgG Subclasses:
IgG is the only isotype that can pass through the human placenta
, thereby providing protection to the fetus
in utero
. Along with IgA
secreted in the breast milk
, residual IgG absorbed through the placenta
provides the neonate with humoral immunity before its own immune system
develops. Colostrum
contains a high percentage of IgG, especially in bovine colostrum
.
IgG can bind to many kinds of pathogen
s, for example virus
es, bacteria
, and fungi, and protects the body against them by agglutination
and immobilization, complement activation
(classical pathway), opsonization for phagocytosis, and neutralization
of their toxins. It also plays an important role in Antibody-dependent cell-mediated cytotoxicity(ADCC) and Intracellular antibody-mediated proteolysis, in which it binds to TRIM21
(the receptor with greatest affinity to IgG in humans) in order to direct marked virions to the proteasome
in the cytosol.
IgG is also associated with Type II and Type III Hypersensitivity
.
IgG antibodies are large molecules of about 150 kDa
composed of four peptide chains. It contains two identical heavy chains of about 50 kDa and two identical light chains of about 25 kDa, thus a tetrameric quaternary structure. The two heavy chains are linked to each other and to a light chain each by disulfide bonds. The resulting tetramer has two identical halves, which together form the Y-like shape. Each end of the fork contains an identical antigen
binding site. The Fc regions of IgGs bear a highly conserved N-glycosylation site. The N-glycans attached to this site are predominantly core-fucosylated diantennary structures of the complex type. In addition, small amounts of these N-glycans also bear bisecting GlcNAc and α-2,6-linked sialic acid residues.
. This emission has been assigned to the breathing motion in the beta-barrel of nine beta-strands in its V
domain. The dynamic mechanism of the "chelate effect" and "trigger effect" of IgG has been analyzed from the angle of low-frequency resonance among the 12 beta-barrels of an IgG molecule.
Note: IgG affinity to Fc receptors on phagocytic cells is specific to individual species from which the antibody comes as well as the class. The structure of the hinge regions gives each of the 4 IgG classes its unique biological profile. Even though there is about 95% similarity between their Fc regions, the structure of the hinge regions is relatively different.
In a model of autoantibody mediated anemia using IgG isotype switch variants of an anti erythrocytes autoantibody, it was found that IgG2a was superior to IgG1 in activating complement. Moreover, it was found that the IgG2a isotype was able to interact very efficiently with FcgammaR. As a result, 20 times higher doses of IgG1 in relationship to IgG2a autoantibodies, were requried to induce autoantibody mediated pathology.
Azeredo et al. J of Exp Med. 2002. 195: 665.
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
molecules. Each IgG is composed of four peptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
chains — two heavy chains γ and two light chains. Each IgG has two antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
binding sites. Other immunoglobulins may be described in terms of polymers with the IgG structure considered the monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
.
IgG constitutes 75% of serum
Blood plasma
Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...
immunoglobulins in humans. IgG molecules are synthesized and secreted by plasma B cells.
Functions
IgG antibodies are involved in predominantly the secondary immune responseMemory B cell
Memory B cells are a B cell sub-type that are formed following primary infection.-Primary response, paratopes, and epitopes:In wake of first infection involving a particular antigen, the responding naïve cells proliferate to produce a colony of cells, most of which differentiate into the plasma...
. The presence of specific IgG, in general, corresponds to maturation of the antibody response. Human IgG Subclasses:
IgG is the only isotype that can pass through the human placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
, thereby providing protection to the fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
in utero
In utero
In utero is a Latin term literally meaning "in the womb". In biology, the phrase describes the state of an embryo or fetus. In legal contexts, the phrase is used to refer to unborn children. Under common law, unborn children are still considered to exist for property transfer purposes.-See also:*...
. Along with IgA
IGA
Iga or IGA may stand for:-Given name:* a female given name of Polish origin. The name originates from the female given name Jadwiga and stands for gia,or gina in the USA....
secreted in the breast milk
Breast milk
Breast milk, more specifically human milk, is the milk produced by the breasts of a human female for her infant offspring...
, residual IgG absorbed through the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
provides the neonate with humoral immunity before its own immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
develops. Colostrum
Colostrum
Colostrum is a form of milk produced by the mammary glands of mammals in late pregnancy. Most species will generate colostrum just prior to giving birth...
contains a high percentage of IgG, especially in bovine colostrum
Colostrum
Colostrum is a form of milk produced by the mammary glands of mammals in late pregnancy. Most species will generate colostrum just prior to giving birth...
.
IgG can bind to many kinds of pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s, for example virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es, bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, and fungi, and protects the body against them by agglutination
Agglutination (biology)
Agglutination is the clumping of particles. The word agglutination comes from the Latin agglutinare, meaning "to glue."This occurs in biology in three main examples:...
and immobilization, complement activation
Complement system
The complement system helps or “complements” the ability of antibodies and phagocytic cells to clear pathogens from an organism. It is part of the immune system called the innate immune system that is not adaptable and does not change over the course of an individual's lifetime...
(classical pathway), opsonization for phagocytosis, and neutralization
Neutralisation (immunology)
Neutralisation in the immunological sense refers to the ability of antibodies to block the site on bacteria or viruses that they use to enter their target cell. One example of this within biology is a neutralizing antibody....
of their toxins. It also plays an important role in Antibody-dependent cell-mediated cytotoxicity(ADCC) and Intracellular antibody-mediated proteolysis, in which it binds to TRIM21
TRIM21
Tripartite motif-containing protein 21 also known as E3 ubiquitin-protein ligase TRIM21 is a protein that in humans is encoded by the TRIM21 gene. Alternatively spliced transcript variants for this gene have been described but the full-length nature of only one has been determined...
(the receptor with greatest affinity to IgG in humans) in order to direct marked virions to the proteasome
Proteasome
Proteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
in the cytosol.
IgG is also associated with Type II and Type III Hypersensitivity
Hypersensitivity
Hypersensitivity refers to undesirable reactions produced by the normal immune system, including allergies and autoimmunity. These reactions may be damaging, uncomfortable, or occasionally fatal. Hypersensitivity reactions require a pre-sensitized state of the host. The four-group classification...
.
Structure
KDA
KDA may refer to:* Karachi Development Authority* Kongsberg Defence & Aerospace* Kotelawala Defence Academy* Kramer Design Associates* Lithium diisopropylamide, KDA is the potassium analogue of lithium diisopropylamideOr kDa may refer to:...
composed of four peptide chains. It contains two identical heavy chains of about 50 kDa and two identical light chains of about 25 kDa, thus a tetrameric quaternary structure. The two heavy chains are linked to each other and to a light chain each by disulfide bonds. The resulting tetramer has two identical halves, which together form the Y-like shape. Each end of the fork contains an identical antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
binding site. The Fc regions of IgGs bear a highly conserved N-glycosylation site. The N-glycans attached to this site are predominantly core-fucosylated diantennary structures of the complex type. In addition, small amounts of these N-glycans also bear bisecting GlcNAc and α-2,6-linked sialic acid residues.
Low-frequency internal motion
Immunoglobulin G has a low-frequency wave number of 28 cm−1 in the Raman spectraRaman spectroscopy
Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...
. This emission has been assigned to the breathing motion in the beta-barrel of nine beta-strands in its V
domain. The dynamic mechanism of the "chelate effect" and "trigger effect" of IgG has been analyzed from the angle of low-frequency resonance among the 12 beta-barrels of an IgG molecule.Subclasses
There are four IgG subclasses (IgG1, 2, 3, and 4) in humans, named in order of their abundance in serum (IgG1 being the most abundant).| Name | Percent | Crosses placenta Placenta The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and... easily |
Complement activator | Binds to Fc receptor Fc receptor An Fc receptor is a protein found on the surface of certain cells - including natural killer cells, macrophages, neutrophils, and mast cells - that contribute to the protective functions of the immune system.... on phagocytic cells >- | IgG1 |
66% | yes (1.47)† | second-highest | >- | 23% | no (0.8)† | third-highest | >- | 7% | yes (1.17)† | highest | >- | 4% | yes (1.15)† | no | intermediate affinity |
| †: Quota cord/maternity concentrations blood. Based on data from a Japanese study on 228 mothers. |
Note: IgG affinity to Fc receptors on phagocytic cells is specific to individual species from which the antibody comes as well as the class. The structure of the hinge regions gives each of the 4 IgG classes its unique biological profile. Even though there is about 95% similarity between their Fc regions, the structure of the hinge regions is relatively different.
In a model of autoantibody mediated anemia using IgG isotype switch variants of an anti erythrocytes autoantibody, it was found that IgG2a was superior to IgG1 in activating complement. Moreover, it was found that the IgG2a isotype was able to interact very efficiently with FcgammaR. As a result, 20 times higher doses of IgG1 in relationship to IgG2a autoantibodies, were requried to induce autoantibody mediated pathology.
Azeredo et al. J of Exp Med. 2002. 195: 665.
See also
- AntibodyAntibodyAn antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
- IgMIGMIGM as an acronym or abbreviation can refer to:* Immunoglobulin M , the primary antibody against A and B antigens on red blood cells* International Grandmaster, a chess ranking* intergalactic medium* Intragroup medium - see: Intracluster medium...
, IgAIGAIga or IGA may stand for:-Given name:* a female given name of Polish origin. The name originates from the female given name Jadwiga and stands for gia,or gina in the USA....
, IgDIGDIGD can refer to:*Internet Gateway Device as defined in UPnP.*İGD, İlerici Gençler Derneği, Progressive Young Association of Turkey*Immunoglobulin D, an antibody protein involved in the maturation of B cells....
, IgEIGEIGE was one of the largest services company buying and selling virtual currencies and accounts for MMORPG. During its peak time, it had offices in Los Angeles, China , and headquarters & customer service centre in Hong Kong. IGE was one of the main monopoly in virtual economy services, also known...
(other heavy chain classes) - B cellB cellB cells are lymphocytes that play a large role in the humoral immune response . The principal functions of B cells are to make antibodies against antigens, perform the role of antigen-presenting cells and eventually develop into memory B cells after activation by antigen interaction...
, Plasma B cell - epitopeEpitopeAn epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that recognizes the epitope is called a paratope...