Severe Traumatic brain injury (sTBI) is a very serious problem, and right now, doctors don't have special treatments to help stop additional injury. Previous studies showed that when a young person gets this kind of brain injury, their body's defense system reacts quickly and strongly. This can cause problems with the brain's protective barrier breaking down and the body making antibodies that attack its own cells. We think that changes in how the brain and the gut (the part of your body that digests food) talk to each other might make these defense reactions stronger. This study wants to figure out exactly how brain injuries change the gut and how that affects the body's defenses. To do this, we will use new ways to study body chemicals and genes \[RNA, cell free DNA (cfDNA) and blood chemistry\] to help us learn why the body reacts this way and how it can lead to more brain injuries and problems in adults with serious brain injuries.