Why Women Get More Autoimmune Diseases

Our curiosity about our ancestors probably doesn't extend to one particular mammal' the first animal to have a placenta after the dinosaur extinction. A new theory suggests that having a placenta contributes to women having more auto immune diseases. In the US, eighty percent of the fifty million Americans with autoimmune disease are women where their own immune systems attack their bodies. For example, up to ninety five percent of people with primary Sj'gren's syndrome are women.

The new theory called the pregnancy compensation hypothesis or PCH states that evolution shaped women's immune systems differently than men. While a woman is pregnant, her immune system is altered in order to protect the woman from microbes and yet refrain from attacking the fetus. But when contraception became available and women had fewer babies, their immune systems may not have adapted.

For millions of years, women had eight to twelve children and in pregnancy, the placenta sends signals to the immune system to not attack the fetus. According to the PCH theory, fewer pregnancies meant fewer negative signals. Over time, the immune system may have become elevated and began attacking the body's own cells. If women with multiple kids really do have lower rates of autoimmune disease, then scientists can use this to chart new paths toward effective therapies.