Final answer:
Microglia are the type of glial cells derived from mononuclear phagocytes, functioning as the CNS's resident macrophages for immune defense.
Step-by-step explanation:
The type of glial cell that is derived from mononuclear phagocytes is called microglia. Microglia in the CNS are indeed smaller glial cells known to originate from macrophages, a type of white blood cell, and integrate into the central nervous system (CNS) during development. They share a common function with macrophages found elsewhere in the body, which is phagocytosis – the process of ingesting and digesting cellular debris, foreign substances, and pathogens. This is why microglia are sometimes also referred to as the resident macrophages of the CNS, acting as the primary immune defense mechanism within the brain and spinal cord.