4. Effector Mechanisms of Humoral Immunity

Question 1

What is the function of plasma S protein?

Answer 1

Inhibits membrane insertion of C5b-C7, prevents the MAC from beginning.

Question 2

What sort of bacteria infect patients with C5, 6, 7, 8, and 9 deficiencies?

Answer 2

Neisseria bacteria

Question 3

What two classes of antibodies can activate the classical compliment pathway?

Answer 3

IgG

IgM

Question 4

What is the function of Factor I regarding the complement system?

Answer 4

Degrades C3b into iC3b, and C3dg.

Breaks C3b into inactive fragments

Question 5

What is the function of Decay Accelerating Factor (DAF)?

Answer 5

Displaces C2a from C4b and Bb from C3b (dissociates both C3 convertases).

Question 6

What is the function of CR1 (CD35)?

Answer 6

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

Question 7

What is the function of FcεRI?

What is the relative affinity for this receptor?

Answer 7

Triggers degranulation in eosinophils

High

Question 8

What class of antibodies is passed via breast milk?

Answer 8

IgA

Question 9

Which Fc receptor “does not mediate effector functions of antibodies?”

What is the affinity for this receptor?

Answer 9

FcγRIIb

Low affinity

pg. 174

Question 10

What is the function of properdin?

Answer 10

Stabalizes C3 convertases on microbial surfaces.

Question 11

What produces C2a and C4b in the lectin pathway?

Answer 11

MASPs (MASP 1, 2, and 3)

Question 12

What three complement deficiencies are associated with Systemic Lupus Erythematosus?

Answer 12

C1q, C2, and C4

Question 13

What is the function of C1 Inhibitor (C1 INH)?

Answer 13

Prevents C1r₂C1s₂ from becoming proteolytically active.

Question 14

What is the function of FcRn?

Answer 14

It is an endosomal receptor in phagocytes and endothelial cells that prevents destruction of IgG, which contributes to its longer half life.

Question 15

Which two IgG subclasses are the most efficient activators of compliment?

Answer 15

IgG3 and IgG1

Question 16

What is the protective mechanism of antibodies following vaccination against pneumococcal pneumonia / Haemophilus?

Answer 16

Opsonization/phagocytosis mediated by IgM and IgG antibodies, and compliment activation.

Question 17

Why might it be beneficial to provide multiple rounds of vaccination?

Answer 17

Because affinity maturation will occur after every vaccination, creating more specific antibodies.

Question 18

What sort of infections are associated with C3 deficiencies?

(What sort of bacteria?)

Answer 18

Pyogenic bacterial infections

Question 19

What is the most common human complement deficiency?

Answer 19

C2 deficiency

Question 20

What Fc receptor is unique to NK cells?

What is its function?

Answer 20

FcγRIII

Antibody dependent, cell mediated, cytotoxicity

Question 21

What is steric hindrance?

Answer 21

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.

Question 22

What phagocyte receptor recognizes the Fc region of IgG1 and IgG3 and triggers phagocytosis?

What is the relative affinity for this receptor?

Answer 22

FcγRI

High affinity

Question 23

What is the function of Factor H?

Answer 23

Binds C3b and displaces Bb, dissolving the C3 convertase of the alternative pathway.

Also helps Factor I cleave C3b

Question 24

What is the function of Mebrane Cofactor Protein (MCP)?

Answer 24

Helps Factor I cleave C3b and C4b

Question 25

Why might it be advantageous to a prolonged immune response to produce more IgG antibodies?

Answer 25

Because IgG antibodies have a half life of about 3 weeks.

Question 26

What is the classical / lectin complement pathway's C3 convertase?

Answer 26

C4bC2a

Question 27

What is bound by C3R and C4R? What are their functions?

Answer 27

iC3b Phagocytosis and leukocyte adhesion to endothelium via ICAM-1

Question 28

What is the function of CR2 receptors?

Answer 28

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.

Question 29

Which subtype of IgG is the least efficient opsonin?

Answer 29

IgG2

Question 30

Which antibody class is responsible for neonatal immunity by being transferred across the placenta?

Answer 30

IgG

Question 31

Why might C2 and C4 deficiencies not be associated with increased rates of infections?

Answer 31

Because the alternative pathway can compensate for the lack.

Question 32

Why can't complement bind to soluble IgM?

Answer 32

IgM undergoes a shape change after it binds, allowing C1 to bind to it.

Question 33

What forces attach complement proteins to microbes?

Answer 33

Covalent attachment

Question 34

What is the function of CD59?

Answer 34

Inhibits Poly-C9 (from the MAC) assembly.

Question 35

Which antibody class is responsible for opsonization?

Answer 35

IgG

Question 36

What are the three subunits of the C1 complement protein? What are their functions?

Answer 36

C1q, C1r, C1s C1q binds to the antibody via its H head C1r and C1s are proteases

Question 37

Why would the C1 complex not activate the complement cascade when binding to one Fc portion of IgG? Why is this important?

Answer 37

Because it must bind to two or more to activate the complement cascade. Prevents complement activating from soluble IgG