This textbook aims to describe in a condensed form the essentials of molecular immunology behind bacterial infections, the microbiome, viral infections (such as influenza and COVID-19), organ transplantations, autoimmunity, allergy and tumor immunology. The book emphasizes the impact of immunology in maintaining our health and preventing disease. Our immune system protects us not only from severe consequences of infectious diseases and getting cancer, but is also able to harm us severely via sepsis, cytokine storms and anaphylactic shocks. Molecular understanding of immunology should allow the reader a more rational handling of common diseases, most of which are associated with chronic inflammation.