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Purpose of Review: Sarcoidosis is an immune-mediated disease of unknown cause. Immune-mediated diseases appear to cluster in patients and in families. We review what is known on this topic for sarcoidosis, and what factors may underlie disease clustering., Recent Findings: In populations of patients with sarcoidosis, relative risk estimates of Sjögren's syndrome, systemic lupus erythematosus, autoimmune hepatitis, ankylosing spondylitis, multiple sclerosis (MS), celiac disease, autoimmune thyroid disease, and ulcerative colitis, varied between 2.1 and 11.6. In relatives of patients with sarcoidosis, relative risk estimates varied between 1.3 and 5.8 for sarcoidosis, MS, celiac disease, type 1 diabetes, Graves' disease, rheumatoid arthritis, Crohn's disease, and ulcerative colitis. Shared risk loci in key immunological pathways provide evidence for a contribution to development of multiple diseases. Identical changes in the immune status, epigenetic alterations, and environmental triggers have been detected in several diseases, and drug-induced disease is likely responsible for a small portion of co-occurring disease., Summary: Clustering of sarcoidosis and other immune-mediated diseases in patients and in their relatives occurs for sarcoidosis, MS, celiac disease, Graves' disease, and ulcerative colitis. Further research is needed to substantiate causal links and risk estimates in patients and their relatives.

Introduction: Although the presence of familial sarcoidosis has been confirmed, clinical and epidemiological data on its characteristics are scattered and sometimes paradoxical. The objective of this review is to assess what is known on the clinical epidemiology of familial sarcoidosis, by combining data from early case reports with recent population based data; aiming to support in clinical decision making and providing information to patients., Method: A systematic review of the literature in PubMed was done and 27 studies with clinical or epidemiological data on familial sarcoidosis, published between 1947 and 2017, were included., Results: The pooled prevalence proportion of familial sarcoidosis, based on twelve study populations, was 9.5% (CI 4.6-16.1), highest in French, African American, Dutch and Irish patients. A heritability of 60-70% was estimated in diverse studies. Relative types and relationships most often reported in familial sarcoidosis were siblings and mother-child relationships. Familial risk is heterogeneous. In African Americans specific environmental factors have been associated with familial sarcoidosis (OR between 1.5 and 3.2). European American and African American subjects had different relative risks for first degree familial relationships (OR of 16.6 vs 3.1) and relative risk differed between relative types. Clinical findings in familial sarcoidosis are still obscure., Conclusions: Prevalence of familial sarcoidosis is high in specific study populations from countries worldwide. The estimated heritability of 60-70%, suggests a shared determinant, and the heterogeneous familial risk, associated with both genetic and environmental factors. Familial relative risks and clinical phenotypes may differ between ethnic groups and relative types, but require further study., (Copyright © 2018 Elsevier Ltd. All rights reserved.)