How do autoimmune diseases cluster in families? A) Familial autoimmune disease. This classical concept indicates the same AD in diverse FDRs. In this case, a proband and a FDR (that is, the father) present with T1D. B) Familial autoimmunity. This new concept corresponds to the presence of different ADs in a nuclear family. C) Multiple autoimmune syndrome. This condition refers to the presence of three or more autoimmune diseases in the same subject. In this case, two brothers met criteria for the syndrome. Moreover, this pedigree also meets criteria for familial autoimmunity. D) Polyglandular autoimmune syndrome type II. In this family, however, familial autoimmune disease and familial autoimmunity coexist. The results of HLA genes (that is, A, B, DRB1, DQB1) typing are shown in colors (by reverse dot blot using InnoLipa Kit). A suggestive linkage among the HLA loci is observed. In these diagrams, people are represented by symbols: circles for female and squares for male, and the bottom line represents the offspring of the couple above. Solid symbols represent affected individuals. Symbol with a diagonal line indicates deceased individual. AdD, Addison's disease; AITD, autoimmune thyroid disease; APS, antiphospholipid syndrome; FDRs, first degree relative; MS, multiple sclerosis; PA, pernicious anemia; SLE, systemic lupus erythematosus; SS, Sjögren's syndrome; T1D, type 1 diabetes; VIT, vitiligo.