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complement membrane attack complex 189 complement receptor 3 (CR3)
ammonia to block the action of C3 and C4; and the adding
of zymosan or cobra venom factor to induce alternate pathway
activation of C3, which consumes C3 in the plasma.
complement membrane attack complex
Refer to membrane attack complex.
complement multimer
A doughnut-shaped configuration as a part of the complement
reaction sequence.
Visible
indicator
system
Three Pathways of Complement System Activation
Parameter Classical Pathway Alternative Pathway lectin Pathway
Activation Binding of antibody molecules
(specifically IgM and IgG1, IgG2,
and IgG3) to foreign particle
Patient serum
being tested for
specific antibody
Invisible
system
No free C
Antibody present
in serum
Ab
C
+
Ag
Sheep RBC + Ab (Hemolysin)
Complement
Known
specific
antigen
No antibody present
in serum
Sheep RBC + Ab
No hemolysis = Positive reaction Hemolysis = Negative reaction
Complement fixation reaction.
complement receptor 1 (CR1)
A membrane glycoprotein found on human erythrocytes,
monocytes, polymorphonuclear leukocytes, B cells, a T cell
subset, mast cells, and glomerular podocytes. On red cells,
CR1 binds C3b or C4b components of immune complexes,
facilitating their transport to the mononuclear phagocyte
system. It facilitates attachment, endocytosis, and phagocytosis
of C3b/C4b-containing complexes to macrophages
or neutrophils and may serve as a cofactor for factor-Imediated
C3 cleavage. The identification of CR1 cDNA
has made possible the molecular analysis of CR1 biological
properties.
+
+
C
+
Free C
Invading microorganisms Binding of MBP to mannose
groups of carbohydrates on
microorganisms
Activation mechanism Antibody-dependent Antibody-independent Antibody-independent
Limb of immunity Adaptive immune response Innate immune response Innate immune response
Components C1 (C1q, C1r, C1s) to C9 Factors B, D, P, H, I C1 (C1r, C1s) to C9
Components that initiate
enzyme cascade
C1 (C1q, C1r, C1s), C2, C4 C3, B, D Lectin, MASP1, MASP2, C4, C2
C3 convertase C4b, C2a, C2b C3bBb C2b, C4b, C2a
C5 convertase C4b, C2a, C3b C3bBb, C3b C4b, C2a, C3b
Terminal components C5, C9, MAC (C5b678(9) n) C5, C9, MAC (C5b678(9) n) C5, C9, MAC (C5b678(9) n)
B = plasma factor B. D = plasma protease factor D. P = properdin. H = protein H. I = factor I. MBP = mannan-binding protein. MASP =
MBP-associated serine protease. MAC: membrane attack complex.
complement receptor 2 (CR2)
A receptor for C3 fragments that also serves as a binding
site for Epstein–Barr virus (EBV). It is a receptor for
C3bi, C3dg, and C3d based on its specificity for their C3d
structures. B cells, follicular dendritic cells of lymph nodes,
thymocytes, and pharyngeal epithelial cells, but not T
cells, express CR2. EBV enters B lymphocytes by way of
CR2. The gene encoding CR2 is linked closely with that of
CR1. A 140-kDa single polypeptide chain makes up CR2,
which has a short consensus repeat (SCR) structure similar
to that of CR1. CR2 may be active in B cell activation.
Its expression appears restricted to late pre-B cells and to
mature B cells. CR2 function is associated with membrane
IgM. Analysis of cDNA clones has provided the primary
structure of CR2.
complement receptor 3 (CR3)
A principal opsonin receptor expressed by monocytes, macrophages,
and neutrophils. It plays an important role in the
removal of bacteria and binds fixed C3bi in the presence of
divalent cations. It also binds bacterial lipopolysaccharides
and β glucans of yeast cell walls. The latter are significant
in the ability of granulocytes to identify bacteria and yeast
cells. CR3 is an integrin type of adhesion molecule that
facilitates the binding of neutrophils to endothelial cells in
inflammation. It enables phagocytic cells to attach to bacteria
or yeast cells with fixed C3bi, β glucans, or lipopolysaccharides
on their surfaces. This facilitates phagocytosis
and the respiratory burst. CR3 is comprised of a 165-kDa
α glycoprotein chain and a 95-kDa β glycoprotein chain.
CR3 appears related to LFA-1 and P-150,95 molecules and
shares a β chain with them. All three of these molecules
are of critical significance in antigen-independent cellular
adhesion, which confines leukocytes to inflammatory areas,
among other functions. Deficient surface expression of
these molecules occurs in leukocyte adhesion deficiency
(LAD), in which patients experience repeated bacterial
infections. The primary defect appears associated with the
common β chain. Besides C3bi binding, CR3 is of critical
significance in IgG- and CR1-facilitated phagocytosis by
neutrophils and monocytes. CR3 has a more diverse function
than does either CR1 or CR2.
C