This short lecture course will provide introduction into exciting field of immunology with special focus on molecular, genetic and cellular mechanisms. Immune system evolved to protect a multicellular organism from invading pathogens, such as viruses, bacteria and fungi. The effective immune system should include molecular and cellular mechanisms for pathogen recognition and elimination. Pathogens usually penetrate through barrier tissues, such as skin or mucosal tissues. Therefore, protection of the barriers is an important strategy of host defense involving specific molecular and cellular mechanisms. When the barriers are damaged the pathogen can get inside, but should be ideally recognized and destroyed, sometimes together with harboring cells of the host organism. All living organisms possess a powerful arm of defense called innate immune system. However, about 5% of them additionally possess the so called adaptive or acquired arm of the immune system which has cellular mechanisms to “remember” the prior encounter with the given pathogen. These two arms of the immune system are utilizing quite distinct molecular and cellular mechanisms, including recognition strategies, but they are actively cooperating to provide optimal protection. Strikingly, bacteria also possess both innate and adaptive immunity that are based on entirely different molecular mechanisms, as compared to eukaryotes. Elucidation of immune mechanisms allows the development of therapeutic strategies, such as vaccinations. Immunology is rapidly progressing field of life sciences with enormous significance for public health. It is not surprising that Nobel prizes are awarded on average every 7 years for the discoveries in the field of immunology.