Early in the pandemic, a team of immunologists from the Max Planck Institute of Immunobiology and Epigenetics in Freiburg and physicians from the University of Freiburg Medical Center joined forces to learn more about immunity in people recovering from COVID infections. The study revealed a yet unknown involvement of Interleukin 33, an important alarm-signal, when immune cells get exposed to SARS-CoV-2 for a second time.