Immune cells cross the inflamed blood–brain barrier. But it’s unclear how brain inflammation begins before immune-cell entry. Studies of a model of multiple sclerosis start to solve this ‘chicken and egg’ conundrum. Multiple sclerosis, a disorder of the central nervous system (CNS), is characterized by damage to myelin, the insulating membrane that surrounds nerve fibres. The lesions found in multiple sclerosis are inflamed, but don’t contain an evident pathogen that might have triggered an immune reaction; hence, it has been proposed that the disorder is caused by an autoimmune process, in which the body’s defence system turns against its own tissue.
Read More: Immunology: In the beginning.