Our immune system is there to protect us from anything that will do harm to our body, such as antigens. However, for someone who have an autoimmune disorder, their immune system fails to identify what is doing harm and helping the body. Based on the U.S. National Library of Medicine,
The exact cause of autoimmune disorders is unknown. One theory is that some microorganisms (such as bacteria or viruses) or drugs may trigger changes that confuse the immune system. This may happen more often in people who have genes that make them more prone to autoimmune disorders.” (“Autoimmune Disorders. U.S. National Library of Medicine.)
There are a couple theories that is developed by experts