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In a study of 972 patients with diabetes mellitus, humoral pancreatic islet-cell antibodies (I.C.Ab.) were detected in highest prevalence in insulin-treated diabetics with (38 per cent) and without (22 per cent) associated overt organ-specific autoimmune disease (A.I.D.) where consideration was not given to the duration of diabetes. They were also detected in 8 per cent of diabetics treated with oral hypoglycemic agents (O.H.A.), but not in diabetics requiring diet alone and in only 0.5 per cent of 434 control subjects. Six per cent of 522 patients with overt organ-specific A.I.D. but not diagnosed to be diabetic had I.C.Ab.s. I.C.Ab.s were present in the sera of 2 per cent of 157 first-degree relatives of I.C.Ab.-positive subjects. In insulin-treated diabetics and, to a lesser extent, in diabetics not requiring insulin, the prevalence of humoral I.C.Ab. was strongly dependent of the duration of the diabetes, being 60 per cent during the first year from diagnosis in the insulin-treated group and falling to 20 per cent at two to five years and to 5 per cent at 10-20 years. The prevalence of I.C.Ab. in insulin-treated diabetics showed no correlation with the patient's age at the time of testing when the duration of diabetes was taken into account. Diabetics who did not require insulin for treatment but who were I.C.Ab.-positive showed a significant tendency to subsequently require insulin and to have a higher prevalence of other autoantibodies than insulin-independent diabetics who were I.C.Ab.-negative. Persistence of I.C.Ab. for more than five years from diagnosis of diabetes was associated with coexistent overt organ-specific A.I.D. and with HLA-B8, A1, and A1 + B8.

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