Viruses and their hosts have gone through a long period of joint evolution. This is why the biological characteristics of the immune system can be studied particularly well by means of viruses. Protective immunity against cytopathogenic viruses, and the escape of non-cytopathogenic viruses from immune control, will be used as examples to describe key characteristics of the immune system, i.e. specificity, the distinction between self and non-self, and immunological memory. They are important in understanding immunobiology, pathogenesis, and the prevention of infectious diseases. Virus – host relationships balanced in different ways can be used to show that immunological specificity and memory are defined best by means of biological equilibria, as the immunological parameters measurable in vitro only rarely reflect genuine, useful information about protective immunity in vivo. Our studies show that the immune system does not distinguish between "self" and "non-self." Instead, B-cells react chiefly to antigen patterns, while T-cells react to antigens only in connection with transplantation molecules as a function of their quantity and localization in the host.