Which component is central to all three complement pathways (classical, lectin, and alternative)?

• A. C3
• B. C5
• C. C1q
• D. Factor B

Answer: (A)

The central idea is that all three complement pathways converge at activation of C3. In the classical and lectin pathways, activation begins with C1 complex or MASP binding to target surfaces, leading to formation of the C3 convertase C4b2a. In the alternative pathway, C3 undergoes spontaneous hydrolysis and, with factor B and D, forms the C3 convertase C3bBb. In all routes, C3 is cleaved into C3a and C3b. C3b deposition drives opsonization and, together with other components, forms C5 convertases to cleave C5 and generate the membrane attack complex, while C3a acts as an inflammatory mediator. Because this activation step is required for all three pathways to propagate the cascade, C3 is the component central to the system. The other options are more pathway-specific or act downstream (for example, C5 appears later; C1q initiates only the classical pathway; Factor B is specific to the alternative pathway).