Complement Biology (CAD-Focused) • The Blood Project

Learning objectives

After completing this quiz, the learner should be able to:

explain why classical pathway activation is central in CAD
distinguish C3-mediated opsonization from MAC-mediated lysis
describe the “hit-and-run” logic of CAD (antibody initiates, complement persists)
interpret common complement labs (especially low C4) mechanistically
connect complement biology to why steroids fail and why proximal complement inhibition works

Which statement best captures the core pathophysiology of CAD?

a

IgG antibodies bind at body temperature and drive splenic clearance

Describes warm AIHA physiology, not CAD.

b

IgM initiates disease, but complement activation determines phenotype

IgM is the trigger, but complement kinetics, amplification, and downstream C3 effects shape anemia and symptoms.

c

Terminal complement lysis is the dominant cause of anemia

MAC-mediated lysis can occur, but is usually not the dominant mechanism.

d

Hemolysis is primarily antibody-mediated without complement involvement

Complement is central in CAD.

Why can relatively small amounts of IgM produce robust complement activation?

a

IgM binds C3 directly and bypasses C1

Classical activation starts at C1, not C3.

b

One pentameric IgM efficiently engages C1q due to clustered Fc regions

Pentameric IgM presents multiple Fc regions in close proximity, making C1q engagement efficient.

c

IgM irreversibly binds red cells at 37°C

gM binding is temperature-dependent and often transient.

d

IgM requires dense clustering like IgG to activate complement

Dense clustering is the IgG problem, not IgM.

Which complement pathway is the dominant initiator in primary CAD?

a

Classical

CAD is fundamentally a classical pathway disease.

b

Alternative

Can amplify downstream, but are not the primary initiators in CAD.

c

Lectin

Can amplify downstream, but are not the primary initiators in CAD.

d

Terminal

“Terminal” is an effector phase, not an initiating pathway.

The key biologic event that drives most hemolysis in CAD is:

a

C3 activation with C3b deposition leading to opsonization

CAD is predominantly a C3-opsonization disease with extravascular clearance (often hepatic).

b

C5b-9 assembly causing explosive intravascular lysis

MAC can contribute in severe exacerbations, but is usually limited.

c

IgM-mediated Fc receptor clearance in the spleen

Fc-mediated splenic clearance is warm AIHA logic.

d

Direct RBC membrane disruption by antibody binding

Antibody binding initiates, but complement effectors do the work.

Why does the DAT in CAD typically detect C3d rather than C3b?

a

C3d is the only fragment that can bind red cells

Not true, C3b covalently deposits on cells.

b

C3b is never formed in CAD

C3b formation is central in CAD.

c

C3d appears only when intravascular hemolysis predominates

C3d positivity does not require intravascular hemolysis.

d

C3b is rapidly processed to iC3b and then to C3d, which persists on surviving cells

C3b is processed during circulation; C3d is a stable remnant and is commonly detected.

What best explains the “hit-and-run” nature of CAD?

a

IgM remains tightly bound at 37°C for hours

IgM binding is often transient and temperature-dependent.

b

Complement activation can continue after IgM disengages, and C3b remains fixed on the RBC surface

Antibody can initiate in cooler beds, then complement amplification and surface C3 persist after rewarming.

c

Hemolysis occurs only at the coldest peripheral temperatures

Consequences can continue centrally after rewarming.

d

Cold avoidance eliminates complement activation in most patients

Cold avoidance helps but does not reliably extinguish activity.

Low C4 with relatively less depressed C3 in a patient with active CAD most strongly suggests:

a

Classical pathway engagement with early consumption of C4

Disproportionately low C4 is a classic signature of classical pathway activation.

b

Primary alternative pathway activation

Would not preferentially consume C4.

c

Complement deficiency unrelated to disease activity

May occur, but the pattern in CAD often reflects ongoing activation.

d

Predominant terminal pathway activation

Terminal activation is downstream and does not explain selective C4 consumption.

Why is intravascular hemolysis usually limited in CAD?

a

IgM cannot activate complement to completion

IgM is highly complement-fixing.

b

Terminal complement is efficiently regulated on RBCs by CD55 and CD59, and many opsonized cells are cleared before MAC forms

CD55/CD59 constrain terminal injury; extravascular clearance often happens before MAC-mediated lysis.

c

CAD red cells are resistant to all complement injury

RBCs are not globally resistant, they are regulated.

d

Intravascular hemolysis requires lectin pathway activation

Not required.

Why do corticosteroids generally fail in primary CAD?

a

They raise complement levels and worsen hemolysis

Not the core mechanistic reason.

b

They reduce C1s activation but not C3 deposition

Steroids do not reliably shut off classical complement activation.

c

They primarily target IgG/Fc-mediated and splenic clearance mechanisms, not IgM/complement-driven opsonization

Steroids target the wrong biology for CAD’s complement-driven pathway.

d

They eliminate the IgM-producing clone quickly

Clonal IgM production is not steroid-responsive in this way.

Which therapeutic statement best aligns with complement biology in CAD?

a

Targeting C1s can rapidly reduce hemolysis by preventing downstream C3 deposition

Proximal classical pathway blockade prevents C3 opsonization and can produce rapid, predictable control.

b

Terminal complement inhibition is always sufficient because MAC drives most anemia

MAC is usually not the dominant mechanism.

c

Complement inhibition works only in severe disease

It works because the mechanism is clean, not because disease is severe.

d

Complement-directed therapy modifies the underlying B-cell clone

Complement inhibition is disease control, not clone modification.

What is the single most important complement effector event in CAD?

Click for Answer

C3 activation with C3b deposition on RBCs, leading to opsonization and predominantly extravascular clearance.

Why is CAD “hit-and-run”?

Click for Answer

IgM may bind transiently in cooler circulation, but complement activation persists and C3 fragments remain on the RBC surface after rewarming.

Sort each feature into the dominant physiologic stream

DAT positivity for C3d on surviving RBCs

Alternative pathway amplification loop after initial C3b

C3b covalently deposits on RBC surface

C1s cleaves C4 and C2 to form C4b2a

CD55 and CD59 limit MAC-mediated lysis

IgM pentamer binding exposes C1q sites

Initiation (classical pathway trigger)

Amplification / opsonization (C3-centered effector)

Terminal / intravascular regulation (MAC and its brakes)

Match each concept to its implication:

Disproportionately low C4

C3d-positive DAT

Rapid response to proximal classical inhibition (C1-level blockade)

Prevention of downstream C3 deposition, halting the dominant effector mechanism

Prior C3b deposition processed to stable remnant on surviving RBCs

Classical pathway engagement with early C4 consumption

Correct! Sorry, Incorrect.

Closing Note

In CAD, the antibody is the spark.

Complement is the fire.

If you follow complement, the rest becomes coherent: why hemolysis is usually extravascular, why the DAT looks the way it does, why symptoms can exceed anemia, why steroids fail, and why proximal complement inhibition works so cleanly.