William Paul, M.D., is the NIH Distinguished Investigator and Chief of the Laboratory of Immunology within the National Institute of Allergy and Infectious Diseases (NIAID). Paul discovered and characterized the cell signaling cytokine IL-4, demonstrating that IL-4 is required for B cell production of IgE, and determined the requirements for CD4+ T cell differentiation. In an interview with JCI Editor-at-Large Ushma Neill, Paul discusses his early research experiences, as well as the influence of Michael Heidelberger on his decision to study immunology. Paul began his training in immunology in Nobel laureate Baruj Benacerraf’s lab at New York University and then moved with Bennacerraf to the NIH in 1968, where he began to focus on T and B cell biology. His lab has served as a training ground for many noted immunologists, including Laurie Glimcher, Mark Davis, and Charlie Janeway.