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Introduction
Autoimmune diseases (ADs) are common (4.5-7%), associated with significant threat to quality of life, and are responsible for substantial direct and indirect societal costs (Autoimmune Diseases Coordinating Committee, 2005; Miller et al. 2012). ADs are a group of chronic inflammatory diseases that develop as a result of the loss of immunological tolerance to self-antigens. ADs afflict specific target organs or multiple organ systems depending on the specific disorder (Anaya, 2012). Interactions between genetic and environmental factors are thought to be the main explanatory mechanisms for their susceptibility and onset (Miller et al. 2012). Still, current known genetic risk factors and environmental exposures are unable to fully explain the occurrence of these diseases.
The literature increasingly suggests a link between depression and physical health, particularly relating to disorders associated with dysfunction of the immune system (Penninx et al. 2013). For example, depression has been found to be associated with increased risk of the onset and progression of chronic diseases such as cardiovascular, neuroplastic, and neurodegenerative diseases (Penninx et al. 2013). Depression is one of the most common diseases globally: 6% of the population meet criteria for major depressive disorder (past year) and one in five individuals experiences a depressive episode during their lifetime (Alonso et al. 2004). As such, depression constitutes a major global problem that has profound social and economic consequences for both individuals and society (Alonso et al. 2004).
To date, few studies have examined the association between depression and ADs. In general, clinical AD patient populations tend to have higher rates of depression compared to those without ADs (Valtysdottir et al. 2000; Carta et al. 2002; Zyrianova et al. 2006; Nery et al. 2007; Gold & Irwin, 2009; Mitsonis et al. 2009; Inal et al. 2010; Sharma et al. 2011; Kirim et al. 2012; Benros et al. 2013; Giynas Ayhan et al. 2014; Schramm et al. 2014) and findings from epidemiologic community-based studies suggest an association between depression and some ADs (Pop et al. 1998; Degner et al. 2001; Carta et al. 2004; Engum, 2007; Maunder & Levenstein, 2008; Bachen et al. 2009; Baysal et al. 2011; Korczak et al. 2011; Vattakatuchery et al. 2011; Roy & Lloyd, 2012; Smith & Gerdes, 2012; van de Ven et al. 2012; Ananthakrishnan