Theresa MacPhail, associate professor of science and technology studies at Stevens Institute of Technology and author of Allergic: Our Irritated Bodies in a Changing World, discusses the origins of  allergies, tracing their discovery back to British physician Charles Blackley who put hay fever on the map. Alanna Collen, evolutionary biologist and author of  10% Human: How Your Body's Microbes Hold the Key to Health and Happiness, explores the link between our microbiomes and the likelihood of developing allergies.