## Pathological Distinction Between Type 1 and Type 2 DM ### Insulitis: The Hallmark of Type 1 DM **Key Point:** Insulitis—characterized by CD8+ T lymphocytic infiltration of pancreatic islets—is the pathognomonic early lesion of Type 1 DM and reflects autoimmune beta-cell destruction. **High-Yield:** Insulitis is present at diagnosis in Type 1 DM but absent in Type 2 DM. This autoimmune infiltrate progressively destroys beta cells, leading to absolute insulin deficiency. ### Comparative Pathology Table | Feature | Type 1 DM | Type 2 DM | |---------|-----------|----------| | **Insulitis** | Present (CD8+ T cells) | Absent | | **Amyloid deposition** | Rare, late | Common, early | | **Beta-cell mass** | Severely reduced (>90% loss) | Mildly reduced (20–50% loss) | | **Fibrosis** | Minimal | Prominent | | **Mechanism** | Autoimmune destruction | Insulin resistance + beta-cell dysfunction | ### Why Insulitis Distinguishes Type 1 1. **Autoimmune pathogenesis**: Type 1 DM is driven by T-cell–mediated destruction of insulin-producing beta cells, resulting in lymphocytic infiltration visible on histology. 2. **Absence in Type 2**: Type 2 DM is characterized by insulin resistance and progressive beta-cell dysfunction, but without autoimmune inflammation. 3. **Timing**: Insulitis is present at or before clinical onset in Type 1 DM, making it the earliest and most specific discriminator. **Clinical Pearl:** In Type 1 DM, up to 90% of beta-cell mass is lost by the time of diagnosis; in Type 2 DM, only 20–50% is lost, and the loss is gradual and non-inflammatory. [cite:Robbins 10e Ch 24]
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