Alternative complement pathway
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What it means
Antibody-independent — spontaneous C3 hydrolysis ("tick-over") amplified on pathogen surfaces (Factor B, Factor D, properdin) → C3 convertase (C3bBb) → amplification loop. Key innate defence before specific antibody exists. Dysregulation (Factor H/I deficiency) → atypical HUS and age-related macular degeneration.
Worked example
Factor H deficiency → uncontrolled alternative-pathway activation → atypical haemolytic uraemic syndrome; eculizumab (anti-C5) treats it.
In the exam
An innate pathway activates directly on a bacterial surface without any antibody, via a spontaneous C3 'tick-over' amplified by properdin.
What settles it
Alternative needs no antibody (C4 normal); classical is antibody/C1-triggered (C4 consumed).
Classically confused with
Classical complement pathway
Source: StatPearls — Complement