Final answer:
Astrocytes are the glial cells responsible for forming the blood-brain barrier, which protects the brain from harmful substances in the blood.
Step-by-step explanation:
The cells that form the blood-brain barrier protecting the brain from harmful substances in the blood are the astrocytes. These glial cells are found in the central nervous system (CNS) and have multiple functions including the regulation of ion concentrations, the uptake and breakdown of certain neurotransmitters, and most notably, the formation of the blood-brain barrier—a membrane that separates the circulatory system from the brain. While other glial cells like oligodendrocytes, microglia, and ependymal cells play important roles in supporting neurons and engaging in immune surveillance among other functions, it is the astrocytes that are crucial for regulating the blood-brain barrier.