One of the functions of the immune system is to protect the body by responding to invading microorganisms, such as viruses or bacteria, by producing antibodies or sensitized lymphocytes (types of white blood cells). Under normal conditions, an immune response cannot be triggered against the cells of one’s own body. In certain cases, however, immune cells make a mistake and attack the very cells that they are meant to protect. This can lead to a variety of autoimmune diseases. They encompass a broad category of related diseases in which the person’s immune system attacks his or her own tissue.