It may come as a surprise that the central nervous system (CNS), the brain and spinal cord, hosts its own specialized active immunity.
The brain and spinal cord are hence not totally immune-privileged. The microglial cells (microglia) are the primary cell type within the immunological community of the CNS and fulfill a diverse set of roles associated with CNS homeostasis and immune defense.
Microglia are brain-resident macrophages forming the first active immune barrier in the central nervous system. Microglia are endowed with spectacular plasticity, allowing them to acquire multiple phenotypes and thereby fulfill their numerous functions in health and disease.
Microglia cells are motile, and within their routine comes the constant patrolling and scanning of the CNS microenvironment and the reaction to danger signals.
Somehow, virtually every physiological state of the brain, basic developmental and physiological processes, appears, in one way or another, to involve this amazing cell type.
Microglia exhibit widely differing functions depending on the stage of life and area of residence within CNS. In the healthy prenatal CNS, microglia exert lifelong support to neurogenesis and nerve wiring, and synapse pruning.
In their role as innate immune cells, microglia play their macrophage “act,” sensing danger signals in the brain tissue microenvironment either caused by pathogens or trauma. Like macrophages, they are phagocytotic, antigen-presenting, and display class II HLA-DR antigens on their surface.
Following years of debate, as regards their true identity, the basic immune cell identity of the microglia was unraveled by their positive immunohistochemistry (IHC)-staining for the macrophage-associated proteins, the allograft inflammatory factor 1 (Iba1/AIF1, HPA049234), the integrin alpha M complement component (ITGAM/CD11, AMAb90911), and the scavenger receptors CD68 (AMAb90874) and CD163 (HPA046404).