Why are autoimmune diseases more common in females?

Fetal cell traffic. Fetal cells cross the placenta and lodge in maternal tissues triggering immune reaction to the paternal antigens. That's why autoimmune diseases are almost exclusive seen in women of reproductive age. If you are wondering about such diseases in men or women who have never been pregnant, the answer is that all of us carry cells from our mothers in our bodies since we were born; they can do the same.