Terminal complement / MAC (C5b–C9)
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What it means
The membrane attack complex: C5b recruits C6–C8 then multiple C9 polymerise → transmembrane pore → osmotic lysis. Critical against Neisseria (thin cell wall).
Worked example
Terminal pathway deficiency (C5–C9) → recurrent Neisserial disease — always check complement after recurrent meningococcal/gonococcal infection. Eculizumab blocks C5 (so vaccinate against meningococcus before starting).
In the exam
A young woman has had three episodes of meningococcal disease; her brother had gonococcal bacteraemia — both have a deficiency in the lytic terminal complement components.
What settles it
Terminal/MAC deficiency → recurrent Neisseria; early-component (C1–C4) deficiency → immune-complex/SLE-like disease and pyogenic infection.
Classically confused with
Classical complement pathway
Source: StatPearls — Complement