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C4d 127 C6 deficiency
Immunofluorescence staining of C4d in peritubular capillaries.
C4d
A 45-kDa molecule produced by factor I cleavage of C4bi
when C4b-binding protein is present. C4d is the molecule
in which Chido and Rodgers epitopes are located. It is also
the location of the C4 α chain’s internal thioester bond. C4d
staining of peritubular capillaries, demonstrable by immunofluorescence,
in renal allotransplants suggests a humoral
antibody component of renal allograft rejection.
C5 (complement component 5)
A component comprised of α and β polypeptide chains
linked by disulfide bonds that react in the complement cascade
following C1, C4b, C2a, and C3b fixation to complexes
of antibody and antigen. The 190-kDa dimeric C5 molecule
shares homology with C3 and C4 but does not possess an
internal thioester bond. C5 combines with C3b of C5 convertase
of either the classical or the alternative pathway. C5
convertases split the α chain at an arginine–leucine bond
at position 74–75, producing an 11-kDa C5a fragment that
has both chemotactic action for neutrophils and anaphylatoxin
activity. It also produces a 180-kDa C5b fragment
that remains anchored to the cell surface. C5b maintains a
structure that is able to bind with C6. C5 is a β 1F globulin
in humans. C5b complexes with C6, C7, C8, and C9 to form
the membrane attack complex (MAC), which mediates
immune lysis of cells. Murine C5 is encoded by genes on
chromosome 2.
C5 convertase
A molecular complex that splits C5 into C5a and C5b in
both the classical and the alternative pathways of complement
activation. Classical pathway C5 convertase is
comprised of C4b2a3b, whereas alternative pathway C5
convertase is comprised of C3bBb3b. C2a and Bb contain
the catalytic sites.
C5 deficiency
A very uncommon genetic disorder that has an autosomal
recessive mode of inheritance. Affected individuals have
only trace amounts of C5 in their plasma and have a defective
ability to form the membrane attack complex (MAC)
necessary for the efficient lysis of invading microorganisms.
They have increased susceptibility to disseminated infections
by Neisseria microorganisms such as N. meningitidis
and N. gonorrhoeae. Heterozygotes may manifest 13 to
65% of C5 activity in their plasma and usually show no
clinical effects of their partial deficiency. C5-deficient mice
have also been described.
C5a
A peptide split from C5 through the action of C5 convertases,
C4b2a3b or C3bBb3b. It is comprised of the
74 amino terminal residues of the C5 α chain. It is a
powerful chemotactic factor and an anaphylatoxin, inducing
mast cells and basophils to release histamine. It also causes
smooth muscle contraction, promotes the production of
superoxide in polymorphonuclear neutrophils (PMNs), and
accentuates CR3 and Tp150,95 expression in their membranes.
In addition to chemotaxis, it may facilitate PMN
degranulation. Human serum contains anaphylatoxin inactivator
that has carboxyl peptidase N properties. It deletes the
C terminal arginine of C5a which yields C5a des Arg.
Although deprived of anaphylatoxin properties, C5a des Arg
demonstrates limited chemotactic properties.
C5a receptor (C5a-R)
A receptor found on phagocytes and mast cells that binds
the anaphylatoxin C5a, which plays an important role in
inflammation. Serum carboxyl peptidase N (SCPN) controls
C5a function by eliminating the C-terminal arginine. This
produces C5a des Arg. Neutrophils are sites of C5a catabolism.
C5a-R is a 150- to 200-kDa oligomer comprised of multiple
40- to 47-kDa C5a-binding components. C5a-R mediates
chemotaxis and other leukocyte reactions.
C5a 74des Arg
That part of C5a that remains following deletion of the
carboxyl terminal arginine through the action of anaphylatoxin
inactivator. Although deprived of the anaphylatoxin
function of C5a, C5a 74des Arg demonstrates limited chemotactic
properties. This very uncommon deficiency of C2
protein in the serum has an autosomal-recessive mode of
inheritance. Affected persons show increased likelihood of
developing type III hypersensitivity disorders mediated by
immune complexes, such as systemic lupus erythematosus
(SLE). Whereas affected individuals possess the C2 gene,
mRNA for C2 is apparently absent. Individuals who are
heterozygous possess 50% of the normal serum levels of C2
and manifest no associated clinical illness.
C5aR
This C5 anaphylatoxin receptor belongs to the G-proteincoupled
receptor family, the chemokine receptor branch
of the rhodopsin family. COS-7 cells and HEK 293 cells
expressing C5aR bind C5a with high affinity. C5aR mRNA
is present in peripheral blood monocytes, granulocytes,
and the myeloid fraction of bone marrow. C5aR message is
present in the KG-1 monocytic cell line.
C5b
The principal molecular product that remains after C5a has
been split off by the action of C5 convertase on C5. It has a
binding site for C6 and complexes with it to begin generation
of the membrane attack complex (MAC) of complement
that leads to cell membrane injury and lysis.
C6 (complement component 6)
A 128-kDa single polypeptide chain that participates in the
membrane attack complex (MAC). It is encoded by C6A
and C6B alleles. It is a β 2 globulin.
C6 deficiency
A highly uncommon genetic defect with an autosomal recessive
mode of inheritance in which affected individuals have
only trace amounts of C6 in their plasma. They are defective
in the ability to form a membrane attack complex (MAC)
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