Question:medium
Which complement component is the first common point between the classical and the alternative complement pathways?
Which complement component is the first common point between the classical and the alternative complement pathways?
Show Hint
Both pathways build different convertases, but they both act on the same substrate first.
Updated On: Jun 24, 2026
- C1
- C2
- C3
- C4
Show Solution
The Correct Option is C
Solution and Explanation
Although the classical and alternative routes of complement begin with completely different recognition events, they are engineered to funnel into a single shared terminal sequence, and the question asks where that merge first occurs. The classical arm uses $C1$ to sense immune complexes and then recruits $C4$ and $C2$ to build its convertase, whereas the alternative arm bypasses $C1$, $C4$, and $C2$ entirely and assembles a separate convertase directly on microbial surfaces. The crucial design feature is that both convertases, despite their different make-up, exist for one purpose: to split $C3$. Once $C3$ is cleaved the two streams become indistinguishable and proceed together through $C5$ and on to the membrane attack complex. Thus the earliest molecule common to both pathways is the third component of complement. \[\boxed{C3}\]
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