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Messenger RNA, or mRNA, functions as a kind of middleman--it carries the directions for making proteins from your DNA to the factories in your cells where the proteins will be assembled. It's a bit like the waiter who writes down your order and takes it to the kitchen, where the cooks will make your meal. Using mRNA to make vaccines, Karikó knew, would be a major departure from the way most vaccines work. They all boost your immune system, but mRNA vaccines would do the job unlike any vaccine before. When you're infected by a virus, it invades certain cells in your body, uses the cells' machinery to make more copies of itself, and then releases the newly made viruses into your blood. These new viruses go off in search of more cells to invade, and the process continues in a cycle. Meanwhile, your immune system is primed to look for anything in your body with a shape that it hasn't seen before. When it encounters something it doesn't recognize, it says: "Hey, there's a new shape floating around. It's probably bad news. Let's get rid of it." Ingeniously, your body is capable of going after both the viruses that are floating freely in your bloodstream and the cells that they have invaded. To defeat the ones in your blood, your immune system makes antibodies that grab onto that specific shape. (The cells that produce antibodies are known as B cells, and the ones that attack infected cells are called killer T cells.) Once you're producing antibodies and T cells, your body also produces memory B cells and memory T cells, which as the name implies, help your immune system remember what the new shape looked like.* * I'm simplifying things somewhat.