Assignment # 13

Question 1

In general terms, what is the role of the complement system?

Answer 1

one of the earliest defense mechanisms activated in response to microbial infections.

Question 2

What is the tissue where most of the complement proteins are synthesized?

Answer 2

Complement proteins are synthesized mainly by the liver

Question 3

What are the two major pathways of complement activation and the pathway to which they both converge .

Answer 3

classical (CP) and alternative (AP) pathways that converge to terminal pathway (TP).

Question 4

What infection are patients susceptible if proteins in the terminal pathway are mutated or deficient?

Answer 4

Deficiency in terminal pathway components —> susceptible to Neisseria sp infections

Question 5

What are the components of the classical pathway?

Answer 5

C1,C2,C3,C4,C5

Question 6

what are the components of the alternative pathway?

Answer 6

C3b (tickover), Factor B, Factor D, Properdin, C3, C5

Question 7

what are the components of the terminal pathway?

Answer 7

C5b, C6, C7, C8,C9 (collectively called the membrane attack complex MAC)

Question 8

Describe the classical pathway with respect to mode of activation

Answer 8

happens after adaptive host defenses have been recruited

IgG or IgM specific for an antigen must be generated to bind the antigen. Antigen binds to antibody at fab region leaving Fc region available for C1 to bind. When C1 binds to two adjoining Fc regions of antigen bound IgM or IgG then the C1s esterase activity is activated

Question 9

In the classical pathway why is IgM-microbe complex more effective than IgG-microbe complex

Answer 9

it is a pentamer

IgM complexes are more efficient because one IgM molecule provide two adjacent Fc regions, the minimum number required for C1 activation. In contrast to IgM, about 600 IgG molecules are required to generate two IgG molecules that are sufficiently close for simultaneous binding by C1

Question 10

In the classical pathway what are the steps leading to the formation of the C3 convertase (CP)

Answer 10

C1 cleaves C4 –> C4a and C4b; C1 cleaves C2 –> C2a and C2b; Forms C4b2a on a microbial surface = C3 convertase (CP) <– cleaves C3 to C3a and C3b

Question 11

In the classical pathway what is the formation of C5 convertase (CP)

Answer 11

C3b + C3 convertase forms C4b2aC3b= C5 convertase (CP) —> cleaves C5 to C5a and C5b

Question 12

In the classical pathway what is the coverage to the terminal pathway leading to the formation of MAC and how are microbes then destroyed by MAC?

Answer 12

C5b binding to microbe surface initiates formation of MAC (C5b,6,7,8,9) in the TP.
Insertion of MAC into the target membrane induces osmotic lysis and death of the microbe (or cell)

Question 13

In the alternative pathway what is the role of C3 tickover?

Answer 13

C3b is derived from circulating C3 in a process that can be referred to as Tickover. C3b is needed following invasion by an infectious agent for prompt mobilization and initiating step of the alternative pathway.

Question 14

In the alternative pathway what is the role of factor B and D?

Answer 14

In the presence of microorganisms, particularly bacteria, the spontaneously generated C3b binds to the microbial surfaces, providing an initiating step for complement activation. This is rapidly followed by the deposition, adjacent to C3b, of another complement protein, Factor B, and its hydrolysis (cleaved) to Ba and Bb by Factor D.

Question 15

In the alternative pathway describe the formation of the C3 convertase (AP)

Answer 15

This C3bBb complex generated on the microbial surface is known as the alternative pathway C3 convertase.

Question 16

In the alternative pathway what is the role of properdin?

Answer 16

The C3 convertase is stabilized by a tetrameric protein, properdin, that extends its half life six to ten fold.

Question 17

In the alternative pathway what makes up the formations of the C5 convertase (AP)

Answer 17

C3b is required for the formation of the alternative pathway C5 convertase (C3bBbC3b), which cleaves C5 to generate C5a and C5b.

Question 18

Finally in the alternative pathway how is MAC and the terminal pathway activated?

Answer 18

The formation of C5b is the initiating step for the formation of the membrane attack complex (C5b,6,7,8,9) , which characterizes the terminal (Common) pathway of complement activation.

Question 19

Activation of either complement pathway generates proteolytic fragments that play a role in the immune response. What are the effects of these proteolytic fragments? each card will go through an effect

Answer 19

1. Opsonin-mediated phagocytosis and elimination of immune complexes (C3b)

Question 20

Effect number 2 of proteolytic fragments

Answer 20

2. Degranulation of mast cells and basophils (C3a, C4a, and C5a) which enhances vascular permeability

Question 21

Effect number 3 of proteolytic fragments?

Answer 21

3. Chemotaxis of neutrophils (C5a)

Question 22

Effect number 4 of proteolytic fragments?

Answer 22

4. Osmotic lysis of bacteria via MAC

Question 23

Effect number 5 of proteolytic fragments?

Answer 23

5. C4b2a forms complex on microbe: C3 convertase (CP)- cleaves C3 to C3a and C3b

Question 24

Effect number 6 of proteolytic fragments?

Answer 24

6. C3bBb forms complex on microbe:C3 convertase (CP)- cleaves C3 to C3a and C3b

Question 25

Effect number 7 of proteolytic fragments?

Answer 25

7. C4b2aC3b= C5 convertase (CP) - cleaves C5 to C5a and C5b

Question 26

Effect number 8 of proteolytic fragments?

Answer 26

8. C3bBbC3b = C5 convertase (AP)- cleaves C5 to C5a and C5b

so basically the convertases in both alternative and classical pathway do the same thing despite that they are generated differently

Question 27

In general terms what is the role of complement regulatory proteins?

Answer 27

protect autologous cells from complement mediated damage
OR
minimized the complement activity and deleterious complement effects on autologous cells

Question 28

In general terms why do complement regulatory proteins not protect xenografts

Answer 28

Because these proteins function in a species-specific manner, a fact that is significant to the field of xenotransplantation (transplant of organs/tissues across different species)

Question 29

What regulatory proteins are unique to the classical pathway?

Answer 29

C1 esterase inhibitor (C1 INH) binds to C1 and prevents spontaneous activation and also inactivates protease kallikrein.

C4bp (C4 binding protein) binds to fluid phase C4b, preventing its attachment to cells and hence prevents formation of C3 convertase (CP) C4b2A or if already formed binds to C4b & promotes its dissociation

Question 30

What are the regulatory proteins that are unique to the alternative pathway

Answer 30

Factor H binds to C3b in the fluid phase-prevents binding to cell surface

Factor H competitively binds with C3b –> dissociation C3bBb aka C3 convertase (if C3bBb formed)

Question 31

What are the regulatory proteins that are common to the classical and alternative pathway?

Answer 31

Decay Accelerating Factor (DAF/CD55)-binds to membrane bound C3b or C4b blocking formation of C3 convertase (AP and CP), or dissociating C3 convertases that formed.

Anaphylatoxin inhibitor (Al) binds C3a, C4a, and C5a. Inhibits binds to CR3a/4a, CR5a on mast cells and basophils.

Factor I- cleaves C4b to C3b to inactive forms in presence of co-factors

Question 32

what are the inhibitors of the membrane attack complex in terminal pathway?

Answer 32

These inhibit formation of MAC on autologous cells
S-protein (vitronectin)
CD59/ Homologous restriction factor (HRF20)

Question 33

List the complement receptors and their effects. Each card will go through one

CR1

Answer 33

phagocytes bind C3b and C4b: opsonin mediated recognition of antigen

RBC binds C3b or C4b: immune complex clearance

Question 34

second receptor? CR2

Answer 34

CR2- on B cells binds C3bi –> mode of Epstein Barr virus infection

Question 35

third receptor? CR3a/4a

Answer 35

CR3a/4a binds C3a and C4a ; mast cells and basophils –> degranulation –> histamine release (CR5a binds C5a on mast cells and basophils –>same effect)

Question 36

Fourth receptor? CR5a

Answer 36

C5a on neutrophils bind C5a. Effect is chemotaxis of neutrophils

on basophils and mast cells: degranulation leading to histamine release

Question 37

What is the rationale for requesting a CH 50 complement test?

Answer 37

One of the most efficient and cost effective ways to assess total classical complement pathway functions.

Question 38

If the results indicate abnormal complement activity, what complement components are targeted for further testing?

Answer 38

Test for C3 and C4 are often used

Question 39

How are the complement and intrinsic coagulation pathway systems linked?

Answer 39

linked via the plasma protein kallikrein, the enzymatic form of the zymogen, pre-kallikrein

Question 40

Explain how complement and intrinsic coagulation pathway linked leads to the production of bradykinin, activation of neutrophils, generation of C5a, and chemotaxis of neutrophils

Answer 40

Kallikrein cleaves kininogen –> kininogen(a) and bradykinin
Bradykinin increases vascular permeability –> critical for inflammation
Cleaves C5 (kallikrein substrate in vitro) —> C5a and C5b. C5a is chemotactic for neutrophils

Question 41

How is Kallikrein inactivated?

Answer 41

C1 INH (Complement regulatory protein) inactivates the protease, kallikrein