Complement

Question 1

Which branch of the immune system is the complement system a part of? Provide one detail about its importance (hint: which pathogen type does it typically defend against?)

Answer 1

Innate immunity. One of the first lines of defense against Gram-negative pathogens

Question 2

Complement activation triggers which two signaling pathways?

Answer 2

Phagocytic and proinflammatory pathways

Question 3

What happens if complement regulation malfunctions? Examples?

Answer 3

Injury to cells, tissues, and organs (e.g., kidney destruction in SLE or hemolytic anemias)

Question 4

List the 3 complement pathways

Answer 4

1. Classic
2. Alternative
3. Mannose-binding lectin

Question 5

At which point do the 3 pathways converge?

Answer 5

Cleavage of C3! C3b is the component kept (Pak taught us that C3a is left as a fluid-phase anaphylatoxin but we don’t need to know that for Laurie’s class. Thought it’d be good review anyway)

Question 6

At which stage do the 3 complement pathways differ? What is different for each of them?

Answer 6

They all differ at the very first step.

Classic: C1 binds the microbe
Alternative: C3 binds the microbe
MBL: MASP1 and MASP2 bind the microbe

Question 7

List the C protein sequence order for the Classical pathway

Answer 7

C1, 4, 2, 3, 5, 6 ,7 ,8 ,9

Question 8

Which C proteins comprise the MAC (membrane attack complex)?

Answer 8

C5b-C9

Question 9

What are the Classical pathway’s C3 and C5 convertases?

Answer 9

C3 convertase = C4b2a
C5 convertase = C4b2a3b

Question 10

List the C protein sequence order of the Alternative pathway

Answer 10

C3, B , C3, C5-C9

Question 11

What are the Alternative pathway’s C3 and C5 convertases?

Answer 11

C3 convertase = C3bBb
C5 convertase = C3bBb3b

Question 12

What does MASP stand for in the mannose-binding lectin pathway?

Answer 12

Mannose-binding lectin-associated serine protease

Question 13

What is the C protein order in the MBL pathway?

Answer 13

MASP1/2, C4, C2, C3, C5-C9

Question 14

What are the C3 and C5 convertases of the MBL pathway?

Answer 14

Same as Classical.
C3 convertase = C4b2a
C5 convertase = C4b2a3b

Question 15

List 3 reasons why we measure complement

Answer 15

1. Detect absence of a non-functional protein
2. Assess consumption of complement
3. Monitor patients on immunosuppressive drugs

Question 16

List 5 functions of complement activation

Answer 16

1. Opsonization for easier phagocytosis
2. Recruit phagocytes
3. Cell activation
4. Immune complex removal
5. Cell lysis

Question 17

Give examples of pathology associated with elevated complement levels. Why do these pathological states occur?

Answer 17

Inflammatory conditions, trauma, acute illness (mycocardial infarction) occur because separate complement components are acute-phase proteins (i.e., C3)

Question 18

What may cause decreased complement levels? Which is the most likely reason?

Answer 18

1. Complement excessively activated recently
2. Complement currently being consumed
3. Most likely because of genetic defect

Question 19

Hypocomplementation can result from _________

Answer 19

Complexing of IgG or IgM antibodies with microbe (called immune-complex). This means that the formation of immune-complexes uses up complement, because it’s trying to clear the immune complexes. So, free circulating complement levels decrease.

Question 20

What’s the time frame for complement levels to return to normal?

Answer 20

It’s quick! 1-2 days after complement activation stops

Question 21

List 3 big disease categories associated with hypocomplementemia

Answer 21

1. Rheumatic diseases with immune complexes
2. Infectious diseases
3. Deficiency of control proteins (e.g., C1 inhibitor deficiency or Factor H deficiency)

Question 22

List 3 types of complement deficiency that causes increased susceptibility to pyogenic (pus-forming) infections

Answer 22

1. Opsonization deficiency (ability to opsonize is messed up)
2. Lytic deficiency (Can’t lyse cells. Maybe MAC complex is compromised)
3. MBL pathway deficiency

Question 23

List 3 diagnostic evaluations of complement proteins

Answer 23

1. Hemolytic method
2. Enzyme immunoassay (EIA)
3. Liposome assay

Question 24

Why would you perform diagnostic evaluations on complement? (Hint: what “indications” are there for testing?)

Answer 24

-Recurrent pyogenic infections
-Angioedema without urticaria (aka hives)
-Autoimmune disorders

Question 25

What values are measured in the classical and alternative pathways?

Answer 25

The following values measure total complement activity:
Classical: CH50
Alternative: AH50

Question 26

Describe the hemolytic method for evaluating total complement activity (CH50 or AH50). Include what the result is associated with and how it’s measured)

Answer 26

Result associated with hemolytic activity of the classic complement pathway

Measured as hemolysis of sheep RBCs sensitized by Ab. Each complement component must be activated (lyse cells). Degree of RBC lysis directly proportional to total classic complement activity in the serum

Question 27

Describe the EIA method for evaluating total complement activity (CH50 or AH50). Include what the result is associated with and how it’s measured)

Answer 27

Result is associated with the functional activity of C1-C9.

Measures amount of polymerized C5b-C9 to indicate functional activity of C1-C9. Uses mAb (monoclonal Ab) specific for MAC neoantigen.

Question 28

Describe the liposome assay method for evaluating total complement activity (CH50 or AH50). Include what the result is associated with and how it’s measured)

Answer 28

Result is associated with total classic complement activity (AH50-specific test measures alternative pathway but only available in specialized labs)

Measures color change due to complement lysis of dinitrophenyl (DNP)-labeled liposomes. G6PD activity correlated with total complement activity (https://pubmed.ncbi.nlm.nih.gov/7720251/)