Which organ is chiefly protected by microglial cells?

Microglial cells are a type of glial cell located in the central nervous system (CNS), primarily the brain and spinal cord. They serve as the primary immune defense in the brain, monitoring its health, responding to injury, and protecting against pathogens and neurodegeneration. When there is a threat, such as inflammation or infection, microglial cells become activated to help restore homeostasis.

The brain has unique protective mechanisms because of its isolated environment, including the blood-brain barrier which limits the entry of pathogens and harmful substances. Microglial cells play a critical role within this setting, acting as the resident macrophages of the CNS. Their primary function is to maintain neuronal health by clearing out cellular debris, dead neurons, and even modulating synaptic connections.

In contrast, the other organs listed—lung, liver, and kidney—are protected by different types of immune cells and do not have microglial cells specifically tasked with their defense. Therefore, the role of microglial cells is intrinsically linked to the protection and maintenance of brain health, making it the correct answer to the question.