The diagnosis and clinical significance of polyautoimmunity.

Autoimmune diseases (ADs) are chronic and heterogeneous conditions that affect specific target organs or multiple organ systems. The chronic nature of these diseases places a significant burden on the utilization of medical care, direct and indirect economic costs, and quality of life. ADs are observed in genetically susceptible individuals in whom their clinical expression is modified by permissive and protective environments occurring over time. These are complex traits, meaning that their inheritance does not follow a single-gene dominant or single-gene recessive Mendelian law, and thus that they are polygenic. ADs are often diagnosed according to classification criteria, however they share similar subphenotypes including signs and symptoms, non-specific autoantibodies and other immune changes, which are prone to taxonomic problems. Polyautoimmunity is defined as the presence of more than one AD in a single patient. When three or more ADs coexist, this condition is called multiple autoimmune syndrome (MAS), which represents the best example of polyautoimmunity as well as the effect of a single genotype on diverse autoimmune phenotypes. Its study will provide important clues to elucidate the common mechanisms of ADs (i.e., the autoimmune tautology).