Autoimmune diseases (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. The close relationship between antiphospholipid syndrome (APS) and systemic lupus erythematosus has been studied throughout the years. However, APS may coexist with several other ADs confirming polyautoimmunity in this systemic disease. Herein, we summarized the common characteristics shared between APS and others ADs in light of the autoimmune tautology (that is, common mechanisms of autoimmune diseases).