Generally, your immune system is supposed to protect you from threats such as infections and illnesses. However, autoimmune diseases lead to your immune system mistakingly attacking the healthy cells,tissue of your body. In the case of UC, it is an autoimmune disease that leads to overactivity of the immune systems- which results in the body attacking and damaging the healthy tissues of the gut. This defense response results in inflammation in the large intestine (as well as other symptoms).