Today, we go to a spot that may be one of the most philosophical places in the universe: the thymus, an organ that knows what is you, and what is not you. Its mood may be existential, but its role is practical — the thymus is the biological training ground where the body learns to protect itself from outside invaders (think: bacteria, coronaviruses). But this training is not the humdrum bit of science you might expect. It’s a magical shadowland with dire consequences.  Then, we’ll leave the thymus to visit a team of doctors who are using this organ that protects you as a way to protect someone… else. Their work could change everything. Special Thanks:  One thousand thanks to Hannah Meyer, Salomé Carcy, Josh Torres, and Cold Spring Harbor Laboratory for showing us a real-life (mouse) thymus for this episode. Special thanks also go to Diane Mathis and Kate Webb. Further reading: Wanna do a little light reading? Here’s the immunology textbook Jenni Punt and Sharon Stranford helped write, including a whole section on that funny little thing called AIRE! Kuby Immunology  The science paper that first described what happens inside the thymus as an, “immunological self shadow”. Radiolab is supported by listeners like you. Support Radiolab by becoming a member of The Lab (https://members.radiolab.org/) today. Our newsletter comes out every Wednesday. It includes short essays, recommendations, and details about special events. Sign up (https://radiolab.org/newsletter)! Follow our show on Instagram, Twitter and Facebook @radiolab, and share your thoughts with us by emailing radiolab@wnyc.org. And, by the way, Radiolab is looking for a remote intern! If you happen to be a creative, science-obsessed nerd who is interested in learning how to make longform radio… Apply! We would LOVE to work with you.  You can find more info at wnyc.org/careers.