L16: Complement Flashcards
Classical pathway activation is initiated after
immune complex formation.
Complement component C1 recognizes the
antigen-antibody complex.
The binding of antibody to antigen induces a conformational change in the
antibody constant region.
Binding of antibody to antigen exposes a site on the Fc portion (of the antibody molecule) that can be bound by
C1, the first complement component of the classical pathway
C1 is a macromolecule that consists of….
C1q (comprised of 6 globular heads and extended tails) in complex with C1r and C1s (the C1qrs complex).
Activation of the C1qrs complex occurs when
at least two of the C1q globular heads are simultaneously bound to antibody………for this to occur, two Fc portions need to be in within close molecular proximity of each other on the antigenic surface.
In contrast to IgG, the pentameric nature of IgM allows
a single molecule of antigen bound IgM to activate C1.
Once C1q is bound to antibody, C1r undergoes a
conformational change and becomes enzymatically active.
…C1r then cleaves ___
C1s, which after cleavage is enzymatically active as well.
Unlike the classical activation pathway, the Lectin pathway is initiated by a protein called
Mannose Binding Lectin (MBL) or ficolins, which are homologous to C1q.
MBL binds to mannose and certain other complex carbohydrates that are found on the surface of many microbial pathogens like…
Candida albicans, a fungus with surface mannose residues
MBL/ficolins are physically associated with two serine proteases, called
MASP-1 and MASP-2 (mannan binding lectin-associated serine protease-1) that are similar to C1r and C1s.
MASP-1 and MASP-2 are similar to
C1r and C1s.
When MBL/ficolins bind to surfaces
MASP-1 and MASP-2 become activated.
_______ are similar in structure to MBL, but have different carbohydrate binding proteins
Ficolins
In the classical pathway, which of the following bind to Fc of the antibody?
A. C1q
B. C1s
C. C1r
A. C1q
Which Ig subtypes can activate complement?
IgG and IgM
Why don’t circulating unbound IgG and IgM activate the complement system?
A conformational change is required in the Fc region that will activate complement. This conformational change only occurs when IgM or IgD is bound to antigen.
What complement proteins recruit phagocytic cells to the site of infection and promote inflammation?
C3a, C5a
Phagocytes with receptors for which complement protein, engulf and destroy the pathogen?
C3b
MAC is composed of
C5b, C6, C7, C8, C9
To what residues does MBL bind
mannose and fucose residues
To what residues does Ficolin bind?
acetylated sugars
Which pathway depends on spontaneous hydrolysis of C3?
Alternative pathway
The removal of antigen-antibody complexes from the circulation depends upon
C3b.
Via C3b, antigen-antibody complexes bind to complement receptors on
circulating red blood cells.
C1 inhibitor?
C1 INH. inhibits C1 by dissociating C1r and C1s from the C1 complex.
Name 5 inhibitors of C3 convertase
Decay-accelerating factor (DAF) C4-binding protein (C4BP) Complement receptor 1 (CR1)
Membrane cofactor protein (MCP) Factor I (I)
Name 3 inhibitors of C5 convertase
Factor I (I) Factor H (H) Complement receptor 1 (CR1)
note: CR1 inhibits both C3 and C5 convertases!
What inhibits MAC formation
CD59
MAC deficiency can lead to
infection with Neisseria spp only
C3b deposition deficiency
Infection with pyogenic bacteria and Neisseria spp. is associated with defects in
Hereditary Angioneurotic Edema is associated with deficiency in
C1 INH. Failure to regulate C1 resulting in fluid accumulation, epiglottal swelling.
Paroxysmal Nocturnal Hemoglobinurea is associated with deficiency in
CD59. Leads to RBC lysis because now RBC’s have nothing to defend against MAC attack.
A deficiency of the early components of complement results in poor clearance of…
immune complexes, resulting in increased immune complex disease.
A deficiency of complement component C9 results in susceptibility to:
A. Pyogenic bacteria. B. Neisseria spp. only. C. Immune complex disease. D. Reduced immune complex clearance. E. No phenotype.
B. Since C9 is part of the MAC, and MAC deficiency results in Neisseria infection
