Objective: To provide evidence that complement (C′)-dependent processes may be involved in Rasmussen’s encephalitis (RE).

Background: RE is a rare, progressive, childhood epilepsy syndrome associated with inflammation and neuronal cell loss in a single cerebral hemisphere. Recent work suggests an autoimmune immunoglobulin (Ig) G-mediated process is important in disease pathogenesis.

Methods: Brain samples from RE and complex partial epilepsy control patients were analyzed immunohistochemically. Sections were stained for IgG and the C′ factors C4, C8, and the membrane attack complex (MAC).

Results: Brain samples from three of five patients with active, progressive RE but neither of two chronic RE nor five control epilepsy patients demonstrated immunoreactivity for IgG, C4, C8, and MAC on discrete patches of cerebrocortical neurons. Intensely activated glial fibrillary acid protein–positive astrocytes were found in areas overlapping these patches.

Conclusion: Focally distributed IgG- and C′-positive neurons were found to colocalize with activated astrocytes, suggesting focal IgG-dependent classical C′ cascade pathway activation with attendant tissue damage in this subset of RE patients. Intraparenchymal C′ activation triggered by pathogenic antibodies may contribute to the development of focal inflammation, neuronal cell loss, and pharmacoresistant seizures in some patients with this disease. This process may be an important component in the initial, active phase of RE.