The blood-brain barrier is a term used for the separation of the fluids of the central nervous system from the bloodstream. As such, it acts as a filter between the two systems, for example allowing the transport of some substances on the molecular level but generally blocking larger objects, such as bacteria, from crossing (in this case, from the blood into the central nervous system).
While generally the blood-brain barrier protects our brains from the nastier things that might infiltrate from our blood, it seems that in some cases it fails to do this.
In 2006, researchers at the National Institutes of Mental Health, along with collaborators at the University of Oklahoma and California State University found that some children exhibited an unusual response to strep throat infections. A press release from 2006 details how strep infections led an antibody (or related enzyme) to cross the blood-brain barrier and apparently attack the functioning of the the brain itself, causing symptoms of obsessive-compulsive disorder in the affected children. This condition, called PANDAS, led to the onset of obsessive-compulsive symptoms in children who had previously shown none. Additionally, lab tests showed that the blood of PANDAS patients had high levels of strep antibodies in about three-quarters of the cases studied and the cerebrospinal fluid of the same patients revealed a corresponding enzyme. Patients without PANDAS showed fewer antibodies and little or none of the related enzyme.
It would seem that, in this case, there is strong evidence that a mental health condition can be caused not only by biological factors, but that the onset of symptoms can be frighteningly quick. While we know that congenital defects and injuries can alter behavior, so apparently can the aftermath of a common childhood infection. In this case, it would seem the vital task of the blood-brain barrier to keep separate the resources (and contaminants) in our bloodstream and nervous system fluids does not work properly. Researchers at NIMH add that while there is evidence that the barrier is being crossed, they (at least as of 2006) have no real idea HOW this is happening and why in only some of the many children who suffer strep infections.
Clearly more research needs to be done in this area. It raises questions not only as to the nature of the blood-brain barrier and its function and integrity, but also problems of what other mental health disorders may be related to infections and conditions that previously seemed unrelated. A greater understanding of this interaction might lead to more numerous treatment options and possible prevention.