A perivascular space, also known as a Virchow–Robin space, is an immunological space between an artery and a vein (not capillaries) and the pia mater that can be expanded by leukocytes. The spaces are formed when large vessels take the pia mater with them when they dive deep into the brain. The pia mater is reflected from the surface of the brain onto the surface of blood vessels in the subarachnoid space. Perivascular cuffs are regions of leukocyte aggregation in the spaces, usually found in patients with viral encephalitis.