About one percent of people in industrialized countries are affected by autoimmune diseases; by now, we know more than 80 auto aggressive diseases. This large group of diseases is characterized by a misguided reaction of the immune system to the body’s own tissue. In autoimmune disease, the immune system is no longer able to differentiate between foreign structures and those of the body itself. Autoimmune diseases may appear with various symptoms and manifestations. In many cases, they resemble the symptoms of other diseases and are very difficult to diagnose and to differentiate. This tutorial provides an introduction to the functions of the immune system, the complex functional system responsible for maintaining the body’s integrity by fending off foreign bodies and continually eliminating anomalous cells. In the first part, the elementary mechanisms of immune reactions that underlie the development of autoimmune diseases are explained. The second part demonstrates the sophisticated laboratory methods that are used to perform the challenging task of diagnosing an autoimmune disease.