Microglia are the major cell type expressing MHC class II in human white matter.

In normal human white matter the predominant cell type expressing MHC Class II is the microglia. This population of cells reacts with the pan macrophage marker, EBM/11, and constitutes about 13% of the glial cell population. The intensity of staining was enhanced and the absolute number of Class II+ microglia increased in normal appearing white matter from multiple sclerosis (MS) brain. As T cell activation in MS may occur in the brain the upregulation of microglia bearing MHC Class II may reflect their function as antigen presenting cells in the development of inflammatory lesions.