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immunoglobulin gene superfamily 372 immunoglobulin M (IgM)
Photomicrograph of immunoglobulin G-producing cells (immuno
peroxidase staining).
link the two γ chains in each IgG molecule. There is only
a 5% difference in amino acid sequence among human γ
chain isotypes, exclusive of the hinge region. Cysteine residues,
which make it possible for inter-heavy (γ) chain disulfide
bonds to form, are found in the hinge area. IgG 1 and
IgG 4 have two inter-heavy chain disulfide bonds, IgG 2 has
four, and IgG 3 has 11. Proteolytic enzymes such as papain
and pepsin cleave an IgG molecule in the hinge region to
produce Fab and F(ab′) 2 and Fc fragments. Four murine
isotypes have also been described. Two exons encode the
carboxyl terminal region of the membrane γ chain. Two
γ chains, together with two κ or λ light chains, fastened
together by disulfide bonds, comprise an IgG molecule.
immunoglobulin gene superfamily
Refer to immunoglobulin superfamily.
immunoglobulin genes
Genes that encode heavy and light polypeptide chains of
antibody molecules are found on different chromosomes
(i.e., chromosome 14 for heavy chain, chromosome 2 for
κ light chain, and chromosome 22 for λ light chain). The
DNA of the majority of cells does not contain one gene
that encodes a complete immunoglobulin heavy or light
polypeptide chain. Separate gene segments that are widely
distributed in somatic cells and germ cells come together
to form these genes. In B cells, gene rearrangement leads
to the creation of an antibody gene that codes for a specific
protein. Somatic gene rearrangement also occurs with the
genes that encode T cell antigen receptors. Gene rearrangement
of this type permits the great versatility of the
immune system in recognizing a vast array of epitopes.
Three forms of gene segments join to form an immunoglobulin
light chain gene. The three types include light chain
variable region (VL), joining (JL), and constant region (CL)
gene segments. VH, JH, and CH as well as D (diversity)
gene segments assemble to encode the heavy chain. Heavy
and light chain genes have a closely similar organizational
structure. There are 100 to 300 Vκ genes, 5 Jκ genes, and a
single Cκ gene on the κ locus of chromosome 2. There are
100 VH genes, 30 D genes, 6 JH genes, and 11 CH genes on
the heavy chain locus of chromosome 14. Several Vλ, six
Jλ, and six Cλ genes are present on the λ locus of chromosome
22 in humans. VH and VL genes are classified as V
gene families, depending on the sequence homology of their
nucleotides or amino acids.
immunoglobulin heavy chain
A 51- to 71-kDa polypeptide chain present in immunoglobulin
molecules that serves as the basis for dividing
immunoglobulins into classes. The heavy chain is comprised
of three to four constant domains, depending upon
class, and one variable domain. In addition, a hinge region
is present in some chains. There is approximately 30%
homology, with respect to amino acid sequence, among the
five classes of immunoglobulin heavy chain in humans.
The heavy chain of IgM is μ; of IgG, γ; of IgA, α; of IgD, δ;
and of IgE, ε. Heavy chain constant regions are responsible
for such effector functions as complement activation and
phagocyte engagement.
immunoglobulin heavy-chain-binding protein (BiP)
A 77-kDa protein that combines with selected membrane
and secretory proteins. It is believed to facilitate their passage
through the endoplasmic reticulum.
immunoglobulin κ chain
A 23-kDa, 214-amino acid residue polypeptide chain
composed of a single variable region and a single constant
region. It is one of the two types of light polypeptide chain
present in all five immunoglobulin classes. Approximately
60% of light immunoglobulin chains in humans are κ
with wide variations of their percentages in other species.
Whereas κ chains are virtually absent in immunoglobulins
of dogs, they comprise the vast majority of murine immunoglobulin
light chains. κ light-chain allotypes in humans
are termed Km1, Km1,2, and Km3.
immunoglobulin chain
A 23-kDa, 214-amino acid residue polypeptide chain with a
single variable region and a single constant region. λ chains
represent one of two light polypeptide chains comprising all
five classes of immunoglobulin molecules. Approximately
40% of immunoglobulin light chains in humans are λ. Wide
variations in percentages are observed in other species.
For example, the great majority of immunoglobulin light
chains in horses and dogs are λ, whereas they constitute
only 5% of murine light chains. Constant region differences
exist among λ light chains of mice and humans, and
the molecules are divided into four isotypes in humans.
A different C gene segment encodes the separate constant
regions defining each λ light chain isotype. The human λ
light chain isotypes are designated Kern–Oz + , Kern + Oz–,
and Mcg.
immunoglobulin light chain
A 23-kDa, 214-amino acid polypeptide chain comprised
of a single constant region and a single variable region; the
chain is present in all five classes of immunoglobulin molecules.
The two types of light chains are designated κ and
λ. They are found in association with heavy polypeptide
chains and in immunoglobulin molecules and are fastened
to these structures through disulfide bonds. Approximately
60% of antibodies in humans contain κ light chains and
40% contain λ light chains.
immunoglobulin-like domain
A 100-amino-acid residue structure found in selected
β-sheet-rich proteins with intrachain disulfide bonds. It
is found in immunoglobulins, interleukin-1 and interleukin-6,
T cell receptors, and platelet-derived growth
factors. Structural regions of proteins that are similar to
the immunoglobulin domain, but are present in various
other proteins.
immunoglobulin M (IgM)
Immunoglobulin M (IgM) comprises 5 to 10% of the total
immunoglobulins in adults and has a half-life of 5 days. It