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C7 (complement component 7) 128 C9 (complement component 9)
and have increased susceptibility to disseminated infections
by Neisseria microorganisms, including gonococci and
meningococci. C6-deficient rabbits have been described.
C7 (complement component 7)
An 843-amino acid residue polypeptide chain that is a
β 2 globulin. C5b67 is formed when C7 binds to C5b and
C6. The complex has the appearance of a stalk with a leaf
type of structure. C5b constitutes the leaf, and the stalk
consists of C6 and C7. The stalk facilitates introduction of
the C5b67 complex into the cell membrane, although no
transmembrane perforation is produced. C5b67 anchored
to the cell membrane provides a binding site for C8 and
C9 in formation of the membrane attack complex (MAC).
N-linked oligosaccharides bind to asparagine at positions
180 and 732 in C7.
C7 deficiency
A highly uncommon genetic disorder with an autosomalrecessive
mode of inheritance in which the sera of affected
persons contain only trace amounts of C7 in the plasma.
Patients have a defective ability to form a membrane attack
complex (MAC) and show an increased incidence of disseminated
infections caused by Neisseria microorganisms.
Some may manifest an increased propensity to develop
immune complex (type III hypersensitivity) diseases such as
glomerulonephritis or systemic lupus erythematosus (SLE).
C8 (complement component 8)
A 155-kDa molecule comprised of a 64-kDa α chain, a
64-kDa β chain, and a 22-kDa γ chain. Disulfide bonds join
the α and γ chains. Noncovalent bonds link α and γ chains
to the β chain. The C5b678 complex becomes anchored to
the cell surface when the γ chain inserts into the membrane’s
lipid bilayer. When the C8 β chain combines with
C5b in C5b67 complexes, the α chain regions change in
conformation from β-pleated sheets to α helixes. The
A
B
C5b678 complex has a limited capacity to lyse the cell to
which it is anchored because the complex can produce a
transmembrane channel. The α chain of C8 combines with
a single molecule of C9, thereby inducing C9 polymerization
in the membrane attack complex (MAC). Genes at
three different loci encode C8 α, β, and γ chains. One third
of the amino acid sequences are identical between C8 α
and β chains. These chains share the identity of one fourth
of their amino acid sequences with C7 and C9. C8 is a β 1
globulin. In humans, the C8 concentration is 10 to 20 µ/mL.
C8 deficiency
A highly uncommon genetic disorder with an autosomalrecessive
mode of inheritance in which affected individuals
are missing C8 α, β, or γ chains. The disorder is associated
with a defective ability to form a membrane attack complex
(MAC). Individuals may have an increased propensity to
develop disseminated infections caused by Neisseria microorganisms
such as meningococci.
C9 (complement component 9)
A 535-amino acid residue single chain protein that binds
to the C5b678 complex on the cell surface. It links to this
complex through the α chain of C8, changes in conformation,
significantly increases its length, and reveals hydrophobic
regions that can react with the cell membrane lipid
bilayer. With Zn 2+ present, a dozen C9 molecules polymerize
to produce 100-nm diameter hollow tubes positioned
in the cell membrane to produce transmembrane channels.
The interaction of 12 to 15 C9 molecules with one C5b678
complex produces the membrane attack complex (MAC).
When viewed by an electron microscope, the pores in the
plasma membrane produced by the poly-C9 have a 110-Å
internal diameter, a 115-Å stalk anchored in the membrane’s
lipid bilayer, and a 100-Å structure above the membrane
that gives an appearance of a doughnut when viewed from
C
C9
C9
C9
C9
C5bα
C5β
C8β
C9
C9
C9
A: Electron micrograph of complement lesions (approximately 100 C) in erythrocyte membranes formed by poly C9 tubular complexes. B: Electron
micrograph of complement lesions (approximately 160 C) induced on a target cell by a cloned cytolytic T lymphocyte (CTL) line. CTL- and natural killer
(NK)-induced membrane lesions are formed by tubular complexes of perforin which is homologous to C9. Therefore, except for the larger internal diameter,
the morphology of the lesions is similar to that of complement-mediated lesions. C: Model of MAC subunit arrangement.
C6, C7
C8 α–γ
C9