This book intends providing an introduction to immunology for undergraduate and graduate students. As we were engaged for a few years in teaching in a Master’s Degree at the University of Huè, Viet Nam, the Authors debated at length what should be emphasized and, on the contrary, what could be left to an eventual personal investigation in a basic course of Immunology. Our first agreement was that the main emphasis should be on immunological strategies, while most of the molecular mechanisms could be left to student exploration. Molecular biology is now so pervasive that everyone easily swims in the sea of molecular mechanisms. Updated information on molecular pathways of the immune system can be obtained on internet without difficulty. By contrast, the peculiar logic of immunological reactions is sometimes not obvious and not easy to grasp. A second shared view was that a schematic drawing could be more informative than a long text. Therefore this book is mainly based on text illustrations, while only an explanatory comment is reported in legends. We wish to thank exquisite Mrs. Jehanne Marchesi for the numerous corrections and suggestions.