A study involving 67 college football players has found that a protein biomarker for traumatic brain injury (S100B) was present in varying degrees in the blood samples of all the players after every game, even though none of them suffered a concussion. This demonstrates that even the most routine hits have some impact on the blood-brain barrier and possibly the brain itself.
Moreover, this protein is viewed as an enemy by the body, which reacts by building antibodies. Some of these antibodies pass through the damaged blood-brain barrier to attack the neurons that produced the protein. It’s suggested that the brain degeneration observed among professional football players could result from an out-of-control immune response, with the blood-brain barrier repeatedly opening during the football season, leaving the brain open to a continuous autoimmune-type attack.
Players with the most head hits had the highest S100B levels and elevated levels of autoimmune antibodies.
 Consequences of Repeated Blood-Brain Barrier Disruption in Football Players. PLoS ONE [Internet]. 2013 ;8(3). Available from: http://dx.doi.org/10.1371/journal.pone.0056805