The mononuclear phagocyte system is part of the immune system. It is comprised of resident cells capable of immune function and phagocytosis. Examples of these cells are Kupffer cells in liver, histiocytes in lymph nodes, Langerhans cells of skin and mucosa, Hofbauer cells in placenta, and microglia in central nervous system.
A glial cell or neuroglia is a cell surrounding a neuron. It is responsible for providing mechanical and physical support, supplying nutrients and oxygen to the neuron, providing electrical insulation between neurons, and maintaining homeostasis. Microglia are a type of glial cell. Furthermore, microglia are resident macrophages located in the brain and the spinal cord. Therefore, their main role is to provide immune defense in the central nervous system. They constantly look for plaques and damaged nerve cells in the CNS. They are also capable of recognizing disease-causing agents in the brain that are able to cross the blood-brain barrier. They then swallow them and act as antigen-presenting cells that activate T cells. Microglia constitute 10-15% of all cells in the brain.1
1 Lawson, L. J., Perry, V. H., & Gordon, S. (1992). "Turnover of resident microglia in the normal adult mouse brain". Neuroscience 48: 405–415.