## **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, ultimately, restoring normal cell function. The complement system can be activated through three pathways: the classical pathway, the lectin pathway, and the alternative pathway, which converge into a common pathway.

## **Why the Correct Answer is Right**
The correct answer, **C. C5**, can be activated in both the common as well as alternative pathways because C5 is a component of the terminal complement pathway, also known as the common pathway. This pathway is common to all three activation routes (classical, lectin, and alternative) and leads to the formation of the membrane attack complex (MAC). C5 is cleaved into C5a and C5b by the C5 convertase, which is generated in both the classical/lectin and alternative pathways. C5a acts as an anaphylatoxin and chemotactic factor, while C5b initiates the assembly of the MAC.

## **Why Each Wrong Option is Incorrect**
- **Option A:** C1q is a component of the classical pathway and the lectin pathway but not directly part of the alternative pathway. It binds to antibodies or lectins to initiate the classical or lectin pathway, respectively.
- **Option B:** C3 is the central component of the complement system and can be activated in all three pathways (classical, lectin, and alternative), but the question asks for a component activated in both common and alternative pathways specifically. While C3 is indeed part of the common pathway and can be activated in the alternative pathway, the specificity to both common and alternative pathways without implying the classical pathway makes C5 a more precise answer.
- **Option D:** Factor B is a component specific to the alternative pathway and not part of the common pathway's initiation but rather forms the alternative pathway C3 convertase with C3b.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that deficiencies in late complement components (C5-C9) are associated with increased susceptibility to Neisseria infections, such as Neisseria meningitidis and Neisseria gonorrhoeae, because these pathogens are usually handled by the terminal complement components to form the MAC.

## **Correct Answer: C. C5**