Complex sentence: Microglia are a type of glial cell that are the resident macrophage-like cells of the brain and spinal cord, and thus act as the first and main form of active immune defense in the central nervous system (CNS).
Split into simpler sentences: Microglia are a type of glial cell located throughout the brain and spinal cord. As the resident macrophage cells, they act as the first and main form of active immune defense in the central nervous system (CNS).