Sympathetic nervous system plays an important role in the relationship between immune mediated diseases

T helper (Th) lymphocytes have been classified into distinct subsets, Th1 and Th2 on the basis of the cytokines they produce. According to the cross-regulatory properties of Th1 and Th2 cells, one would assume that to be affected by a Th1 type disease increases susceptibility to a Th1 type disease and inhibits a Th2 type disease and vice versa about being affected by a Th2 type disease. However, the pattern of related diseases does not necessarily follow the conventional pattern of inhibitory effects of Th1 and Th2 immune responses on each other. For example, Mycobacteria including BCG, that induce Th1 immune responses; can modulate some Th1 type autoimmune diseases including MS, experimental autoimmune encephalomyelitis (EAE; an animal model for Multiple Sclerosis) and insulin-dependent diabetes mellitus (IDDM) thereby leading to an alleviation of their symptoms. Also BCG precipitates a syndrome similar to systemic lupus erythematosus (SLE), a Th2 type disease; in NOD mice. The coexistence of the major Th2-mediated atopic diseases such as asthma, eczema and allergic rhinitis with the Th1-mediated autoimmune conditions including; coeliac disease (CD), IDDM, rheumatoid arthritis (RA) and psoriasis is another example that is in apparent disagreement with counter-regulatory effects of Th1/Th2 phenotypes. SNS can be stimulated by pro-inflammatory cytokines, production of which is induced by mycobacteria including BCG. Although these cytokines can inhibit SNS activity in the site of inflammation and secondary lymphoid organs, they increase sympathetic tone in other places. Increased sympathetic tone can induce an anti-inflammatory and Th2 type milieu. This milieu can inhibit MS and IDDM and provide a susceptible environment for starting of SLE. Atopic diseases are Th2 type immune mediated diseases; therefore, they increase the production of Th2 type cytokine and decrease production of pro-inflammatory cytokines in the site of allergic reaction and also in secondary lymphoid organs. Therefore, atopic diseases decrease sympathetic tone in all tissues except in the sites of allergic reaction and secondary lymphoid organs. Decreased sympathetic tone results in a pro-inflammatory milieu and in such an environment, Th1 type autoimmune diseases can affect tissues.