Which agent would be most effective in preventing bystander lysis of red blood cells?

• A. C1-INH
• B. DAF
• C. Factor B
• D. Factor H

Answer: (B)

Preventing bystander lysis involves stopping complement from assembling its membrane attack complex on host cell surfaces. Decay-accelerating factor acts directly at the cell surface to shut down the C3 and C5 convertases (C4b2a and C3bBb), accelerating their dissociation. This reduces deposition of C3b and prevents the downstream steps that lead to MAC formation, protecting red blood cells from lysis even when the complement system is activated. While inhibiting classical pathway initiation (C1 inhibitor) or dampening the alternative pathway (Factor H) can help, DAF provides a broader, surface-level defense against complement activation on host cells, making it the most effective choice in this context. Factor B is a component that promotes the alternative pathway convertase, so it would not prevent lysis.