Geoepidemiology of Autoimmune Diseases

Among the over 100 human diseases considered to be autoimmune, all share a hereditary component but genetic factors are insufficient to explain the susceptibility to immune tolerance breakdown and the clinical variability observed in autoimmunity. This is well mirrored by the largely incomplete concordance rates of autoimmune diseases between monozygotic twins, which share identical genome. Genome-wide association studies have identified numerous genetic polymorphisms associated with autoimmune diseases, in some cases within genes shared among diseases, thus suggesting a common mechanistic pathway, which may indicate a genetic susceptibility resulting from the additive effects of several common risk variants. The association between autoimmunity and environmental factors has been suspected for many years, particularly with epidemiological evidence. The “exposome” may measure the effects of environmental factors, as the term encompasses all putative nonhereditary factors, both exogenous and endogenous and the microbiota, the ecological community of commensal, symbiotic and pathogenic microorganisms living in exquisite relationship with us. From both experimental and theoretical viewpoints, the infections by viruses, bacteria, parasites, and fungi are ideal triggers for autoimmunity, also due to the individual susceptibility to develop acute and chronic infections and to the parallel geoepidemiology of autoimmunity and infections worldwide.