There is growing evidence suggesting that hormones, including sex hormones, can affect and be affected by the immune system. For example, both estrogen and progesterone, two important female sex hormones, may suppress some immune activity. When these hormone levels are higher during pregnancy, women with MS tend to have less disease activity. Testosterone, the primary male hormone, may also act as an immune response suppressor. The higher levels of testosterone in men may partially account for the fact that more than twice as many women as men have MS.