How does it happen that a healthy person reacts by forming high-affinity antibodies only to infectious agents that are dangerous to the body, but not to its own antigens, nor to foreign antigens that do not threaten it? This is a question that has been controversial for more than a hundred years. The problem is far from being purely theoretical. The answer depends on what treatment approach will be chosen for autoimmune diseases. Over the past decade, it has been convincingly shown that T- and B- lymphocytes with antigen specificity are present in every healthy individual. Our data confirm that the reactivity of IgG in normal human serum to antigens in liver extract, determined by Western blot, is weak. However, IgG purified from the same serum in identical concentration recognizes multiple liver antigens. The mere presence of autoreactive cells and antibodies is not sufficient to trigger an autoaggressive disease. The boundary between physiological autoreactivity and autoaggressive immune response is not sharp and easy to delineate. Lymphocytes that recognize autoantigens are under strict control in healthy individuals. The monograph is devoted to the mechanisms of this control. Some unusual hypotheses and models aimed at unraveling the mystery of autoimmune diseases are also examined.