4. Effector Mechanisms of Humoral Immunity Flashcards
What is the function of plasma S protein?
Inhibits membrane insertion of C5b-C7, prevents the MAC from beginning.
What sort of bacteria infect patients with C5, 6, 7, 8, and 9 deficiencies?
Neisseria bacteria
What two classes of antibodies can activate the classical compliment pathway?
IgG
IgM
What is the function of Factor I regarding the complement system?
Degrades C3b into iC3b, and C3dg.
Breaks C3b into inactive fragments
What is the function of Decay Accelerating Factor (DAF)?
Displaces C2a from C4b and Bb from C3b (dissociates both C3 convertases).
What is the function of CR1 (CD35)?
It is a receptor for C3b and C4b, and promotes phagocytosis of C3b- and C4b- coated particles as well as clearance of immune complexes from circulation.
Functions like MCP as a cofactor for C3b cleavage into iC3b
What is the function of FcεRI?
What is the relative affinity for this receptor?
Triggers degranulation in eosinophils
High
What class of antibodies is passed via breast milk?
IgA
Which Fc receptor “does not mediate effector functions of antibodies?”
What is the affinity for this receptor?
FcγRIIb
Low affinity
pg. 174
What is the function of properdin?
Stabalizes C3 convertases on microbial surfaces.
What produces C2a and C4b in the lectin pathway?
MASPs (MASP 1, 2, and 3)
What three complement deficiencies are associated with Systemic Lupus Erythematosus?
C1q, C2, and C4
What is the function of C1 Inhibitor (C1 INH)?
Prevents C1r2C1s2 from becoming proteolytically active.
What is the function of FcRn?
It is an endosomal receptor in phagocytes and endothelial cells that prevents destruction of IgG, which contributes to its longer half life.
Which two IgG subclasses are the most efficient activators of compliment?
IgG3 and IgG1
What is the protective mechanism of antibodies following vaccination against pneumococcal pneumonia / Haemophilus?
Opsonization/phagocytosis mediated by IgM and IgG antibodies, and compliment activation.
Why might it be beneficial to provide multiple rounds of vaccination?
Because affinity maturation will occur after every vaccination, creating more specific antibodies.
What sort of infections are associated with C3 deficiencies?
(What sort of bacteria?)
Pyogenic bacterial infections
What is the most common human complement deficiency?
C2 deficiency
What Fc receptor is unique to NK cells?
What is its function?
FcγRIII
Antibody dependent, cell mediated, cytotoxicity
What is steric hindrance?
The process by which antibodies bind to structures such as hemagglutinin (influenza virus) and pili (gram negative bacteria) and prevent them from contacting the host cell.
What phagocyte receptor recognizes the Fc region of IgG1 and IgG3 and triggers phagocytosis?
What is the relative affinity for this receptor?
FcγRI
High affinity
What is the function of Factor H?
Binds C3b and displaces Bb, dissolving the C3 convertase of the alternative pathway.
Also helps Factor I cleave C3b
What is the function of Mebrane Cofactor Protein (MCP)?
Helps Factor I cleave C3b and C4b
Why might it be advantageous to a prolonged immune response to produce more IgG antibodies?
Because IgG antibodies have a half life of about 3 weeks.
What is the classical / lectin complement pathway’s C3 convertase?
C4bC2a
What is bound by C3R and C4R?
What are their functions?
iC3b
Phagocytosis and leukocyte adhesion to endothelium via ICAM-1
What is the function of CR2 receptors?
Bind to inactivated complement products iC3b, C3dg, and C3d and act as a coreceptor for B Cell activation.
Is also a receptor for Epstein-Barr virus to enter B Cells and infect them.
Which subtype of IgG is the least efficient opsonin?
IgG2
Which antibody class is responsible for neonatal immunity by being transferred across the placenta?
IgG
Why might C2 and C4 deficiencies not be associated with increased rates of infections?
Because the alternative pathway can compensate for the lack.
Why can’t complement bind to soluble IgM?
IgM undergoes a shape change after it binds, allowing C1 to bind to it.
What forces attach complement proteins to microbes?
Covalent attachment
What is the function of CD59?
Inhibits Poly-C9 (from the MAC) assembly.
Which antibody class is responsible for opsonization?
IgG
What are the three subunits of the C1 complement protein?
What are their functions?
C1q, C1r, C1s
C1q binds to the antibody via its H head
C1r and C1s are proteases
Why would the C1 complex not activate the complement cascade when binding to one Fc portion of IgG?
Why is this important?
Because it must bind to two or more to activate the complement cascade.
Prevents complement activating from soluble IgG
