## **Core Concept**
The complement system is a part of the innate immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promoting inflammation and enhancing the ability of the immune system to eliminate pathogens. The alternative complement pathway is one of the three pathways that activate the complement system.

## **Why the Correct Answer is Right**
The correct answer, **C. Factor H and Factor I (and properdin) but particularly C3b**, indicates that the activator of the alternative complement pathway involves **C3b**, which is a fragment of the complement component C3. In the alternative pathway, the spontaneous hydrolysis of C3 and subsequent binding of C3b to factor B, forms a fluid-phase C3 convertase (C3bBb), which then stabilizes and becomes active on microbial surfaces. **Properdin** (also known as factor P) stabilizes this convertase.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately represent an activator of the alternative complement pathway.
- **Option B:** This option is incorrect as it likely refers to components not directly associated with the activation of the alternative pathway.
- **Option D:** This option is incorrect because it does not specify the correct activator.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Factor H** is a regulatory protein that inhibits the alternative pathway by acting as a cofactor for **Factor I** in the degradation of C3b and preventing the formation of the C3 convertase. Deficiencies in **Factor H** are associated with atypical hemolytic uremic syndrome (aHUS), a condition characterized by uncontrolled activation of the alternative complement pathway.

## **Correct Answer:** C. C3b.