It is very interesting to look back on the history of health care and, in particular, chiropractic. Today when people think of chiropractic care they automatically think of management for back and neck pain. In fact, until very recently back pain made up only a small percentage of chiropractic patients. A majority of chiropractic patients into the 1950s were seeking care for infectious diseases. If you talk to an "old school" chiropractor that was practicing during this time, they would tell you that they were focused on treating patients with Polio. It was clear that chiropractic care increased the function of the immune system, but the mechanism for this was not well understood until very recently (1997).
While in chiropractic school I read a book entitled "Flu" by Gina Kolata. This book documented the Spanish Flu epidemic of 1918. This book states that the Spanish Flu was the largest epidemic in all of recorded history. In fact, over the course of just 6 months, 100 million people lost their lives as a result of this virus. To put that into perspective, that is more people than were killed in all of history's recorded wars, COMBINED!
During this time chiropractic was still in it's infancy. The first chiropractic adjustment was given in 1895 making chiropractic around 20 years old at the time of this outbreak. However, Doctors of Chiropractic stood up to this large task at hand and took care of those infected. What they found was those that chose chiropractic management had 1/40th the death rate compared to those that chose allopathic, drug management. It is for this reason that chiropractors were licensed. It is very interesting that chiropractors were licensed due to their ability to manage infectious diseases such as the Spanish Flu.
So how exactly does a chiropractic adjustment increase the efficiency of the immune system? In 1997 a study came out that proved when you mechanically stimulate a spinal ligament (as is done in a chiropractic adjustment) it sends a signal into the spinal cord which stimulates the brain. This study also proved that during this process it also stimulates the sympathetic nervous system which is responsible for the function of our immune system. This is why those under regular chiropractic care experience a increase in immune system function.