The history of this text started years ago after reading Wittgenstein¿s ¿Tratactus Logico-Philosophicus¿. At some time later, it seemed to me a good idea to follow the ¿tratactus¿ structure to attempt to write a minimal description of the immune system. I finally did it for fun and hopefully to be useful to whomever reads it.The text reflects my own personal view of the vertebrate¿s immune system (IS). It is centered on concepts and ideas that were developed since 1986 based on work from my own lab1 and from Benedita Rocha¿s lab2 and I¿m greatly indebted to her for this. I have kept it short and focused on what I believe are the essential features of the IS. I¿ve tried to avoid too much detail and most of the complex immunology jargon. If some now fashionable aspects of the IS are only superficially mentioned it is because I feel that they may be not so relevant after all. Perhaps for all these reasons I give no detailed sources and simply refer the reader to some general inspiring non