Biosensors　based　on　the　principle　of　surface　plasmon　resonance　(SPR)　are　surface-sensitive　optical　devices　used　for　monitoring　biomolecular　interactions　at　the　sensor　surface　in　real　time　without　any　labeling.　It　is　used　in　a　wide　variety　of　areas　including　proteomics,　clinical　diagnosis,　environmental　monitoring,　drug　discovery,　and　food　analysis.　C-reactive　protein　(CRP)　is　a　marker　of　inflammation,　which　undergoes　conformation　changes　in　local　lesions,　leading　to　the　formation　of　mCRP.　Autoantibodies　against　mCRP　are　frequently　detected　in　systemic　lupus　erythematosus　(SLE)　and　associated　with　disease　activity　and　prognosis.　An　SPR　immunoassay　for　CRP　autoantibodies　at　complement　factor　H-CRP　interface　is　described　in　this　chapter.