“It’s a very well-done study that convincingly shows there is a tonsil-like structure” in the fish, says immunologist and cell biologist Anna Huttenlocher of the University of Wisconsin-Madison, who wasn’t connected to the research. A second study published today in Science Immunology suggests bony fish—the group that includes trout, zebrafish, and salmon—possess another key part of the human immune system that enables certain immune cells to fine-tune defensive proteins known as antibodies.