Autoimmune Diseases Overview Autoimmune diseases are health conditions that happen when your immune system attacks your body instead of defending it. Healthcare providers sometimes call them autoimmune disorders. Usually, your immune system is like your body’s built-in security system. It automatically detects substances that shouldn’t be in your body (like viruses, bacteria or toxins) and sends out white blood cells to eliminate them before they can damage your body or make your sick. If you have an autoimmune disease, your immune system is more active than it should be. Because there aren’t invaders to attack, your immune system turns on your body and damages healthy tissue. Autoimmune diseases are chronic conditions. This means if you have an autoimmune disease, you’ll probably have to manage it and the symptoms it causes for the rest of your life. Types of autoimmune diseases There are more than 100 different autoimmune diseases. They can affect almost any tissue or organ in ...