If you think the stress you experience during the holidays is doing you harm, you're right. Scientists from the University of Connecticut and Yale University say that entertaining your in-laws or traveling long distances this time of year can make you sick, and they they know why.According to their study published in the Journal of Leukocyte Biology, the same part of our nervous system that is responsible for the fight-or-flight response (called the sympathetic nervous system) also controls regulatory T cells, which are used by the body to end an immune response once a foreign invader has been removed or destroyed.To make this discovery, the research team injected mice with a drug called 6-hyroxydopamine (6-OHDA) that selectively removes sympathetic nerves located in different organs, or a saline solution. Mice injected with 6-OHDA, which effectively severed the link between the nervous system and the immune system had twice as many regulatory T cells as the control group in their spleens and lymph nodes.Further analysis showed that the increase in regulatory T cells resulted from an increase in a protein called "TGF-beta," which directs the development and survival of regulatory T cells. With this information in hand, the researchers sought to see if 6-OHDA would prevent autoimmune disorders from developing.To do this, they injected 6-OHDA or a saline solution into mice before subjecting them and a control group to conditions known to cause an autoimmune disease similar to multiple sclerosis in humans. Unlike the control group, the mice treated with 6-OHDA did not develop the autoimmune disease, showing that not only can the sympathetic nervous system negatively affect the immune system, but it also shows how it might be possible to prevent or stop autoimmune disorders."Ever since Hans Seyle's groundbreaking work on stress, scientists have been trying to understand why stressful situations often exacerbate autoimmune diseases and cause re-emergence of latent infections," said John Wherry, Ph.D., Deputy Editor of the Journal of Leukocyte Biology. "In true fight or flight situations, stress can be a lifesaver, but understanding how the neurological response to the stress of everyday events such as seeing your family around the holidays impacts immune responses should provide opportunities for new therapies.": Sourojit Bhowmick, Anurag Singh, Richard A. Flavell, Robert B. Clark, James O'Rourke, Robert E. Cone, 'The sympathetic nervous system modulates CD4+FoxP3+ regulatory T cells via a TGF-β-dependent mechanism', Journal of Leukocyte Biology, 2009, doi: 10.1189/jlb.0209107