Philippa Marrack is one the world’s most influential immunologists. With John Kappler, her husband and collaborator, she has made fundamental discoveries about the immune system’s T cells, helping to explain their ability to recognize infection, their involvement in allergy, and their role in autoimmune diseases such as juvenile diabetes, lupus, and arthritis. Dr. Marrack also identified and named “superantigens,” potent immunological activators that induce toxic shock and other illnesses.