The pathogenesis of Alzheimer's disease (AD) includes the participation of the immune system. To identify antigenic targets in AD, we screened a human microvascular endothelial cell cDNA library with sera from patients with AD, and we identified rabaptin 5 (RABPT5). We detected serum IgG specific to RABPT5 in 65% of patients with AD and in 35% of patients with systemic lupus erythematosus, but in no healthy controls. Our results demonstrated a massive redistribution of this protein in the cytoplasm of endothelial and neuronal cells in apoptosis. In conclusion, we identified RABPT5 as a novel autoantigen in AD.