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-immunological references.

I look forward to raising in the general non-scientific reader an interest for an immune system where lymphocytes are mainly "concerned" with replication, survival factors and homing to the appropriate niche. This is the 1960s "sex, drugs and rock'n roll" view of the IS. Moreover, there are many concepts that are shared with other fields, e.g., ecology, economics.

I hope to stimulate quite a lot of discussion among those that study the Immune System. The text opens opportunities on Immunology teaching by focusing on concepts, interactions and their relatedness and all those as one. The readers may build frameworks of cross-references between statements that are not in line to create alternative reading paths. They should interact with each other to compare interpretations and refer to the immunology literature. They may create new connections, add new sub-sections, references and suggest modifications. To the medical doctor or the advanced specialist the text encapsulates the Immune System and provides a novel prism with which to approach Immunology. By attempting to always follow a logical line of thought, I end up by making new statements, some of which remain hypothetical, waiting for experimental testing, that change the current views of the IS. The purpose was that "each" statement should force the reader to stop, think and whenever possible, test. By doing so, I hope to provoke new questions and inspire new experimental approaches and research.

While working on this manuscript and looking for inspiration, I played many games of Shanghai II. Sometimes I got the cookie "Wise men learn much from fools..." There are many "fools" in science. Dear reader, please be wise.

Antonio A. de Freitas, MD, PhD