Well, because our healthy cells produce induced proteins called self antigens and they display these self antigens on special protein complexes found on the membrane of those healthy cells and they display these antigens to wide blood cells. So when a wide blood cell sees a self antigen on a healthy cell, it will not attack that healthy cell. So under normal and healthy conditions, our immune system has no problem distinguishing between these self antigens found on healthy cells and pathogenic antigens found on infected cells or on invading pathogens such as bacterial cells. However, in certain cases and in certain individuals, our immune system can actually lose its natural immunological tolerance to the healthy cells of our body. And what that means is our immune system can no longer distinguish or differentiate between self antigens of healthy cells and pathogenic antigens. And at this point, the white blood cells will begin to attack not only the pathogens and infected cells, but also the healthy cells of our body.