The prelude to type-1 diabetes is leukocyte infiltration into the pancreatic islets, or insulitis. This process begins in pancreatic lymph nodes when T lymphocytes reactive to islet beta cells encounter antigen-presenting cells (APCs) displaying peptides derived from beta cell proteins. We show here that a ripple of physiological beta cell death, which occurs at 2 wk of age in all mouse strains, precipitates the arrival of such APCs, and that the relevant APC is a dendritic cell of CD11c+CD11b+CD8alpha- phenotype. These findings have significant implications concerning the nature of the diabetes-provoking deficits in NOD mice, the identity of the primordial diabetogenic antigens, and our understanding of the balance between immunity and tolerance in a pathological context.