The immune system plays an important role in lung diseases. As autoimmune diseases are rare in diagnostic pathology, and patients usually have been investigated in the clinics, our opinion was rarely requested. But this has changed: Immuno-oncologic treatment is now part of the treatment options in lung cancer, and resistance mechanisms are becoming an important question in pathologic diagnostic work-up, our knowledge in immunology is requested for interpretation of tissue reactions. In this chapter some basic information about immunology is presented. The intent is to provide the reader with some basics to understand the mechanisms within our tissue samples. In addition, these basic aspects in immunology will also help in understanding immune diseases, not only autoimmune ones but also allergies. As our knowledge of the immune system is rapidly expanding, and even specialists might have problems covering all aspects of immunology, the chapter tries to simplify the mechanisms and the involved cells as far as possible. For in deep understanding, the reader is referred to the many books on immunology.