Which of the following is characteristic of complement components?

• A. Heat stable.
• B. Normally present in serum.
• C. Present as active enzymes.
• D. Mainly synthesized by B cells.

Answer: (B)

Complement components are a set of proteins that circulate in blood serum as inactive precursors and are ready to be activated when needed. Being normally present in serum means they’re already around in the bloodstream, forming a rapid, amplifiable cascade that helps fight infection through opsonization, inflammation, and direct cell lysis. They’re not active enzymes by default; activation converts them into active proteases step by step to drive the cascade. They’re not primarily produced by B cells; most complement proteins are synthesized by the liver and released into circulation. This combination—present in serum as inactive precursors and activated upon demand—best fits the characteristic described.