## **Core Concept**
The membrane attack complex (MAC) is a critical component of the complement system, which is part of the innate immune system. It is responsible for creating pores in the membranes of target cells, leading to cell lysis and death. The formation of MAC is a key mechanism by which the complement system helps eliminate pathogens.

## **Why the Correct Answer is Right**
The correct answer, , refers to the terminal complement components C5b, C6, C7, C8, and multiple molecules of C9 that assemble to form the membrane attack complex. This complex integrates into the target cell membrane, creating a large pore that disrupts membrane integrity, leading to an influx of water and ions, and ultimately causing cell lysis. The process begins when C5 is cleaved into C5a and C5b; C5b then binds to the surface of the target cell, recruiting C6 and C7, followed by C8, and finally multiple molecules of C9.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because C1 through C4 are involved in the recognition and initiation of the complement cascade, not in forming the membrane attack complex.
- **Option B:** This option is incorrect because while C3b is crucial for the opsonization of pathogens and the amplification of the complement cascade, it is not a component of the MAC.
- **Option D:** This option is incorrect because C5a is an anaphylatoxin that acts as a potent chemotactic factor for neutrophils but is not part of the MAC; it is the C5b fragment that participates in MAC formation.

## **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 infections, particularly with *Neisseria* species (e.g., *Neisseria meningitidis*), because MAC plays a critical role in eliminating these pathogens.

## **Correct Answer:** C. C5b-C9.