The lymph nodes were known in antiquity—you can see them without a microscope—and were first described in Peri Adenon (On Glands), the Hippocratic treatise that has been described as a “milestone” in the history of immunology.” But the rest of the lymphatic system was more inscrutable. It wasn’t until relatively recently that science really began to understand the lymph system. We are, in fact, still uncovering some of the secrets of this crucial part of our physiology.