It’s one of life’s little ironies. The proteins in our bodies fight infection, carry messages, ferry oxygen and build tissue. But then, like double agents in a spy novel, they can betray us. They overreact to a virus and attack our own organs. They promote cancer, help clog arteries or set up roadblocks in the brain. We may never know until symptoms appear — a lump, chest pain, severe memory lapses — and irreversible damage is done.