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C3e 126 C4c
C3e
A C3c α chain nonapeptide that causes leukocytosis.
The peptide is comprised of
Thr–Leu–Asp–Pro–Glu–Arg– Leu–Gly–Arg.
C3f
A 17-amino acid residue peptide split from the α chain of
C3b by factor I, with factor H or complement receptor 1
present.
C3g
An 8-kDa molecule comprised of the amino terminal,
47-amino acid residues of C3dg. Trypsin digestion of C3dg
yields C3g, whose function is unknown.
C3H/HeJ mouse
A mutant substrain of C3H mice that manifests a suppressed
response by macrophages and B cells to challenge with
lipopolysaccharide. Their macrophages do not produce
interleukin-1 (IL1) and tumor necrosis factor (TNF) following
the lipopolysaccharide challenge. This mutation has an
autosomal-dominant mode of inheritance and is encoded by
genes on chromosome 4. This immunosuppression leads to
an increased incidence of microbial infections in these mice.
C4 allotype
Complement component 4 (C4) is encoded by two genes
designated C4A and C4B, located within the major histocompatibility
complex (MHC) class III region on the short
arm of chromosome 6 in humans. Each locus has numerous
allelic forms, including nonexpressed or null alleles. C4A
is usually Rodgers-negative and Chido-negative, whereas
C4B is usually Rodgers-negative and Chido-positive. C4
protein is highly polymorphic with more than 40 variants
that include null alleles (C4Q0) at both loci. Null alleles are
defined by the absence of C4 protein in plasma and exist
in normal populations at frequencies of 0.1 to 0.3%. The
presence and number of null alleles determine the expected
reference range of serum C4.
C4 (complement component 4)
A 210-kDa molecule comprised of α, β, and γ chains. The α
chain has an internal thioester bond linking a cysteine residue
and adjacent glutamate residue. C4 reacts immediately
following C1 in the classical pathway of complement activation.
C1s splits the α chain of C4 at position 76–77, where
an arginine–alanine bond is located. This yields a 8.6-kDa
C4a fragment, an anaphylatoxin, and C4b, which is a larger
molecule. C4b remains linked to C1. Many C4b molecules
can be formed through the action of a single C1s molecule.
Enzymatic cleavage renders the α chain thioester bond of
the C4b fragment very unstable. The chemically active form
of the molecule is termed metastable C4b. C4bi intermediates
form when C4b thioester bonds and water molecules
react. C4b molecules may become bound covalently to cell
surfaces when selected C4b thioester bonds undergo transesterification,
producing covalent amide or ester bonds with
proteins or carbohydrates on the cell surface. This enables
complement activation to take place on the surfaces of cells
where antibodies bind. C4b may also link covalently with
antibody. C4 is first formed as a 1700-amino acid residue
chain that contains α, β, and γ chain components joined
through connecting peptides. C4A and C4B genes located
at the major histocompatibility complex on the short arm
of chromosome 6 in humans encode C4. Slp and Ss genes
located on chromosome 17 in the mouse encode murine C4.
C4 deficiency
An uncommon genetic defect with an autosomal-recessive
mode of inheritance. Affected individuals have defective
classical complement pathway activation. Those who
manifest clinical consequences of the defect may develop
systemic lupus erythematosus (SLE) or glomeruloneophritis.
Half of the patients with C4 and C2 deficiencies develop
SLE, but deficiencies in these two complement components
are not usually linked to increased infections.
C4a
A 76-amino acid terminal residue peptide produced by
C1s cleavage of C4. Together with C3a and C5a, C4a is an
anaphylatoxin that induces degranulation of mast cells and
basophils associated with histamine release and the features
of anaphylaxis. However, the anaphylatoxin activity of C4a
is 100 times weaker than those of the other two anaphylatoxin
molecules.
C4A
A very polymorphic molecule expressing the Rodgers
epitope encoded by the C4A gene. The equivalent murine
gene encodes a sex-limited protein (SLP). It has less
hemolytic activity than does C4B. C4A and C4B differ in
only four amino acid residues in the C4d region of the α
chain. C4A is Pro–Cys–Pro–Bal–Leu–Asp, whereas C4B is
Leu–Ser–Pro–Bal–Ile–His.
C4b
The principal molecule produced when C1s splits C4. C4b
is that part of the C4 molecule that remains after C4a has
been split off by enzymatic digestion. C4b unites with
C2a to produce C4b2a, an enzyme known as the classical
pathway C3 convertase. Factor I splits the arginine– asparagine
bond of C4b at position 1318–1319 to yield C4bi, if
C4b binding protein is present. C4b linked to particulate
substances reacts with complement receptor 1.
C4B
A polymorphic molecule that usually expresses the Chido
epitope and is encoded by the C4B gene. The murine
equivalent gene encodes an Ss protein. It shows greater
hemolytic activity than does C4A.
C4b-binding protein (C4bp)
A 600-kDa protein in serum capable of binding six C4b
molecules at once by means of seven spokes extending from
a core at the center. C4b halts progression of complement
activation. Factor I splits C4b molecules captured by C4bp.
C4bp belongs to the regulators of complement activity molecules.
C4bp interferes with C2a association with C4b. It
also promotes C4b2a dissociation into C4b and C2a and is
necessary for the action of factor I in splitting C4b to C4bi
and of C4bi into C4c and C4d. The C4bp gene is located on
chromosome 1q3.2.
C4b inactivator
Refer to factor I.
C4bi (iC4b)
When factor I splits C4b, C4bi is the principal product of the
reaction. When C4b-binding protein is present, C4bi splits
an α chain arginine–threonine bond to yield C4c and C4d.
C4c
The principal product of factor I cleavage of C4bi when
C4b-binding protein is present. This 145-kDa molecule of
unknown function is comprised of β and γ chains of C4 and
two α chain fragments.