Autoimmune thyroid disease--Hashimoto thyroiditis and Graves' disease--patients produce high levels of thyroid autoantibodies and contain lymphoid tissue that resembles secondary lymphoid follicles (LFs). We compared the specificity, structure, and function of tonsil and lymph node LFs with those of the intrathyroidal LFs to assess the latter's capability to contribute to autoimmune response. Thyroglobulin and thyroperoxidase binding to LFs indicated that most intrathyroidal LFs were committed to response to thyroid self-antigens and were associated to higher levels of antibodies to thyroglobulin, thyroperoxidase, and thyroid-stimulating hormone receptor. Intrathyroidal LFs were microanatomically very similar to canonical LFs, ie, they had well-developed germinal centers with mantle, light, and dark zones and each of these zones contained B and T lymphocytes, follicular dendritic and interdigitating dendritic cells with typical phenotypes. Careful assessment of proliferation (Ki67) and apoptosis (terminal dUTP nick-end labeling) indicators and of the occurrence of secondary immunoglobulin gene rearrangements (RAG1 and RAG2) confirmed the parallelism. Unexpected high levels of RAG expression suggested that receptor revision occurs in intrathyroidal LFs and may contribute to generate high-affinity thyroid autoantibodies. Well-formed high endothelial venules and a congruent pattern of adhesion molecules and chemokine expression in intrathyroidal LFs were also detected. These data suggest that ectopic intrathyroidal LFs contain all of the elements needed to drive the autoimmune response and also that their microenvironment may favor the expansion and perpetuation of autoimmune response.