In the ongoing struggle between the human immune system and the cancers that learn to evade it, researchers at Keio University have identified a quiet molecular accomplice: D-serine, a naturally occurring amino acid variant that appears to help gastric tumors construct a chemical sanctuary against immune attack. Published in August 2026, the findings reveal that elevated D-serine in the blood not only marks an immunosuppressive tumor environment but also predicts which patients will fail to respond to checkpoint inhibitor therapy. What began as a question about mirror-image molecules has opene