## **Core Concept**
Complement proteins are a part of the innate immune system and play a crucial role in defending against pathogens. They constitute a significant portion of the serum proteins. The complement system consists of over 20 proteins that work together to help eliminate pathogens from an organism.

## **Why the Correct Answer is Right**
The complement system proteins make up approximately 10% of the serum proteins. This is significant because it highlights the substantial role these proteins play in the immune response. The complement proteins are produced by the liver and are constantly present in the blood, ready to be activated in response to infection.

## **Why Each Wrong Option is Incorrect**
* **Option A:** 1-2% - This is too low considering the critical role complement proteins play in the immune system and their relatively high concentration in serum.
* **Option B:** 5-6% - While closer, this still underestimates the actual percentage of serum proteins that are complement proteins.
* **Option D:** 20-25% - This overestimates the percentage; complement proteins are significant but do not constitute as high as 20-25% of serum proteins.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that complement proteins are crucial for the opsonization of pathogens, activation of the inflammatory response, and direct killing of certain bacteria. Deficiencies in complement proteins have been associated with increased susceptibility to infections and autoimmune diseases.

## **Correct Answer:** C. 10%.