## **Core Concept**
The central nervous system (CNS) comprises the brain and spinal cord, and it has a unique set of immune cells known as microglia. These cells act as the primary immune cells and scavengers within the CNS.

## **Why the Correct Answer is Right**
Microglia are a type of glial cell located throughout the brain and spinal cord. They act as the first and main form of active immune defense in the CNS. Microglia are known to function as scavenger cells, removing pathogens, dead neurons, and other debris through a process called phagocytosis. They express various surface receptors that help them recognize and engulf foreign particles and cellular debris.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Astrocytes are a subtype of glial cells in the brain or spinal cord. They are not primarily involved in immune responses or acting as scavenger cells; instead, they provide nutrients to neurons, maintain the blood-brain barrier, and have other supportive functions.
- **Option B:** Oligodendrocytes are responsible for myelination of neurons in the CNS, which is crucial for the rapid transmission of electrical signals along the axons of neurons. They do not act as scavenger cells.
- **Option D:** Ependymal cells line the cerebrospinal fluid (CSF) spaces in the CNS, including the ventricles and central canal of the spinal cord. They are involved in the production and circulation of CSF but do not function as scavenger cells.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that microglia can become activated in response to injury or disease within the CNS, leading to the release of various cytokines and chemokines that can either promote neuroinflammation or contribute to tissue repair. Dysregulation of microglial function has been implicated in several neurodegenerative diseases.

## **Correct Answer:** . Microglia