When looking at the role of the immune system, it is hard to discount the major role that the nervous system plays in regulation. From direct nerve / innervation of the immune system in the thoracic spine to decreasing stress responses in the body, your brain is the driving force behind whether or not your immune system responds properly to the bacteria, viruses, and other pathogens we fight on a daily basis.
In fact, a chiropractic study completed in 1994 showed a 48% increase in CD4 cells in patients suffering from HIV. CD4 cells play a major role in stimulating your immune system to fight off invading pathogens by telling other immune cells what to do and how to respond. Metaphorically speaking, they are the quarterback calling the play to the other immune cells.
Decreasing nerve interference allows your body to function better and improve health overall, especially in those that are immune compromised.
Hightower BC, Pfleger B, Selano J. THE EFFECTS OF SPECIFIC UPPER CERVICAL ADJUSTMENTS ON THE CD4 COUNTS OF HIV POSITIVE PATIENTS. CRJ. 1994 ;3(1):32-39