The Extra X Chromosome and Autoimmune Susceptibility in Klinefelter Syndrome

Objective To systematically evaluate and quantitatively synthesize the association between Klinefelter syndrome (KS) (47,XXY karyotype) and autoimmune susceptibility.Methods PubMed/MEDLINE, Scopus, and Web of Science were systematically searched from database inception to April 30, 2026, for studies evaluating autoimmune diseases or autoimmune serological markers in individuals with KS. The protocol was registered in PROSPERO (CRD420261399097). Random-effects models were prespecified, summary data were extracted from published reports, and pooled effect estimates were expressed as odds ratios (ORs) with 95% CIs. Subgroup analyses were performed according to outcome definition (isolated autoantibody positivity vs clinically manifest autoimmune disease). Risk of bias was assessed using the Newcastle-Ottawa Scale.Results Eight studies were included in the qualitative synthesis and six studies (8 datasets) in the quantitative analysis, comprising 2289 individuals with KS and 51 250 114 controls. KS was associated with markedly increased autoimmune susceptibility compared with control populations (OR 23.25, 95% CI 9.96-54.31), with moderate-to-substantial between-study heterogeneity (I2 = 66.4%). Associations were stronger for clinically manifest autoimmune diseases (OR 48.46, 95% CI 17.68-132.87) than for isolated autoantibody positivity (OR 9.99, 95% CI 1.18-84.77). The autoimmune spectrum predominantly involved female-predominant and interferon-associated disorders, including systemic lupus erythematosus, Sj & ouml;gren syndrome, systemic sclerosis, autoimmune thyroid disease, and type 1 diabetes mellitus.Conclusion KS is associated with increased autoimmune susceptibility, particularly for clinically overt autoimmune diseases. These findings support a potential contribution of sex chromosome complement to immune dysregulation and reinforce the importance of clinical awareness of autoimmune comorbidities in individuals with KS.