Somewhere between 24 and 50 million Americans have an autoimmune disease, a condition in which the immune system attacks our own tissues. As many as 4 out of 5 of those people are women. Rheumatoid arthritis, multiple sclerosis and scleroderma are examples of autoimmune disorders marked by lopsided female-to-male ratios. The ratio for lupus is 9 to 1; for Sjogren’s syndrome, it’s 19 to 1. Stanford Medicine scientists and their colleagues have traced this disparity to the most fundamental feature differentiating biological female mammals from males, possibly fostering a better way to predict autoimmune disorders before they develop.