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c allotype
A rabbit immunoglobulin λ light chain allotype 100 designated
as c7 and c21.
C domain
Refer to constant domain.
C gene
DNA encoding the constant region of immunoglobulin
heavy and light polypeptide chains. The heavy chain C gene
is comprised of exons that encode the different homology
regions of the heavy chain.
C gene segment
DNA coding for a T cell receptor or an immunoglobulin
polypeptide chain constant region. One or more exons
may be involved. Constant region gene segments comprise
immunoglobulin and T cell receptor gene loci DNA
sequences that encode TCR α and β chains and nonvariable
regions of immunoglobulin heavy and light polypeptide
chains.
C region
Refer to constant region.
C region (constant region)
Abbreviation for the constant region carboxyl terminal portion
of immunoglobulin heavy or light polypeptide chain
that is identical in a particular class or subclass of immunoglobulin
molecules. C H designates the constant region of
the heavy chain of immunoglobulin, and C L designates the
constant region of the light chain.
C segment
An exon that encodes an immunoglobulin molecule’s constant
region domain.
s
R
S
R
C1 Complex
C1 Complex.
C
C1
A 750-kDa multimeric molecule comprised of one C1q, two
C1r, and two C1s subcomponents. The classical pathway of
complement activation begins with the binding of C1q to
immunoglobulin M (IgM) or IgG molecules. C1q, C1r, and
C1s form a macromolecular complex in a Ca 2+ -dependent
manner. The 400-kDa C1q molecule possesses three
separate polypeptide chains that unite into a heterotrimeric
structure resembling a stem that contains an amino terminal
in triple helix and a globular structure at the carboxyl
terminus resembling a tulip. Six of these tulip-like structures
with globular heads and stems form a circular and
symmetric molecular complex in the C1q molecule that has
a central core. The serine esterase molecules designated C1r
and C1s are needed for the complement cascade to progress.
These single chain 85-kDa proteins unite in the presence
of calcium to produce a tetramer comprised of two C1r and
two C1s subcomponents to form a structure that is flexible
and has a C1s–C1r–C1r–C1s sequence. When at least two
C1q globular regions bind to IgM or IgG molecules, the C1r
in a tetramer associated with the C1q molecule becomes
activated, leading to splitting of the C1r molecules in the
tetramer with the formation of 57- and a 28-kDa chains.
The latter, termed C1r, functions as a serine esterase, splitting
the C1s molecules into 57- and 28-kDa chains. The
28-kDa chain derived from the cleavage of C1s molecules,
designated C1s, also functions as a serine esterase, cleaving
C4 and C2 and causing progression of the classical complement
pathway cascade.
C1 deficiencies
Only a few cases of C1q, C1r, or C1r and C1s deficiencies
have been reported. The deficiencies have an autosomal-recessive
mode of inheritance. Patients with these defects may
manifest systemic lupus erythematosus (SLE), glomerulonephritis,
or pyogenic infections. They exhibit increased
incidence of type III (immune complex) hypersensitivity
diseases. Half of C1q-deficient persons may contain physiologic
levels of mutant C1q that are not functional.
C1 esterase inhibitor
A serum protein that counteracts activated C1. This diminishes
the generation of C2b which facilitates development
of edema.
C1 inhibitor (C1 INH)
A 478-amino acid residue, single polypeptide chain protein
in the serum. It blocks C1r activation, prevents C1r cleavage
of C1s, and inhibits C1s splitting of C4 and C2. The
molecule is highly glycosylated, with carbohydrates making
up approximately one half of its content. It contains seven
O-linked oligosaccharides linked to serine and six N-linked
oligosaccharides tethered to an asparagine residue. Besides
its effects on the complement system, C1 INH blocks factors
in the blood clotting system, including kallikrein, plasmin,
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C