There has been another fascinating immune system discovery. Bone marrow, the spongy tissue inside most of our bones, produces red blood cells as well as immune cells that help fight off infections and heal injuries. Apparently some small tunnels that connect the bone marrow of the skull with brain tissue. These tunnels appear to be heretofore unknown pathways that permit immune cells to travel directly from the skull to a damaged part of brain tissue. When a disorder such as a stroke occurs in the brain, this tunnel apparently allows immune system cells to be sent directly to the site of injury in the brain instead of having to wait for cells from a bone-marrow site in the body to reach brain tissue.