The textbook provides essential information on all aspects of structure and functions of the immune system under normal and pathological conditions. The knowledge about innate immunity and the mechanisms of adaptive immunity formation is presented at the organismal, cellular, and molecular levels. Information is provided about the immune cell populations, the structure of immune receptors, the intracellular signaling pathways, major histocompatibility complex (MHC), immunity genetic control, and immunological memory. The immune deficiencies, allergic and autoimmune diseases are reviewed. An overview of clinical use of immunotropic drugs is presented. The textbook is recommended for medical students. Updated second edition.