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Large polypeptide subunit of an antibody From Wikipedia, the free encyclopedia
The immunoglobulin heavy chain (IgH) is the large polypeptide subunit of an antibody (immunoglobulin). In human genome, the IgH gene loci are on chromosome 14.
A typical antibody is composed of two immunoglobulin (Ig) heavy chains and two Ig light chains. Several different types of heavy chain exist that define the class or isotype of an antibody. These heavy chain types vary between different animals. All heavy chains contain a series of immunoglobulin domains, usually with one variable domain (VH) that is important for binding antigen and several constant domains (CH1, CH2, etc.). Production of a viable heavy chain is a key step in B cell maturation. If the heavy chain is able to bind to a surrogate light chain and move to the plasma membrane, then the developing B cell can begin producing its light chain.[2]
The heavy chain does not always have to bind to a light chain. Pre-B lymphocytes can synthesize heavy chain in the absence of light chain, which then can allow the heavy chain to bind to a heavy-chain binding protein.[3]
There are five types of mammalian immunoglobulin heavy chain: γ, δ, α, μ and ε.[4] They define classes of immunoglobulins: IgG, IgD, IgA, IgM and IgE, respectively.
Each heavy chain has two regions:
Cows, specifically Bos taurus, show a variation on the general mammalian theme in which the heavy chain CDR H3 region has adapted to produce a divergent repertoire of antibodies which present a "stalk and knob" antigen interaction surface instead of the more familiar bivalent tip surface.[7] The bovine CDR is unusually long and contains unique sequence attributes which support the production of paired cysteine residues during somatic hypermutation.[7] Thus, where in humans the somatic hypermutation step targets the V(D)J recombination process, the target in cows is on the creation of diverse disulfide bonds and the generation of unique sets of loops which interact with antigen.[7] A speculated evolutionary driver for this variation is the presence of a vastly more diverse microbial environment in the digestive system of the cow as a consequence of their being ruminants.[7]
Jawed fish appear to be the most primitive animals that are able to make antibodies like those described for mammals.[8] However, fish do not have the same repertoire of antibodies that mammals possess.[9] Three distinct Ig heavy chains have so far been identified in bony fish.
Similar to the situation observed for bony fish, three distinct Ig heavy chain isotypes have been identified in cartilaginous fish. With the exception of μ, these Ig heavy chain isotypes appear to be unique to cartilaginous fish. The resulting antibodies are designated IgW (also called IgX or IgNARC) and IgNAR (immunoglobulin new antigen receptor).[13][14] The latter type is a heavy-chain antibody, an antibody lacking light chains, and can be used to produce single-domain antibodies, which are essentially the variable domain (VNAR) of an IgNAR.[15][16][17] Shark single domain antibodies (VNARs) to tumor or viral antigens can be isolated from a large naïve nurse shark VNAR library using phage display technology.[16][18]
IgW has now also been found in the group of lobe finned fishes including the coelacanth and lungfish. The IgW1 and IgW2 in coelacanth has a usual (VD)n-Jn-C structure as well as having a large number of constant domains.[19][20]
Frogs can synthesize IgX and IgY. [21]
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