Complex sentence: Immune-mediated diseases are conditions caused by abnormal activity of the body's immune system in which it over-reacts (for example, immune-mediated contact dermatitis) or starts attacking the body (for example, autoimmune hemolytic anemia).
Split into simpler sentences: Immune-mediated diseases are conditions which result from abnormal activity of the body's immune system. The immune system may over-react (for example, immune-mediated contact dermatitis) or start attacking the body (for example, autoimmune hemolytic anemia).