In a fascinating study published in the January 26, 2005 issue of The Journal of the American Medical Association, researchers in Australia showed that people who had younger siblings in the first six years of their life had less risk of developing multiple sclerosis. They calculated how many infant years of exposure a person spent with siblings less than 2 years of age during the first six years of his or her life. People who had 1 to 3 years of exposure had about a 43% lesser chance of developing multiple sclerosis, compared to someone with no younger siblings. Those who had 3 to 5 years of exposure had 60% less risk, and those who had 5 or more years had 88% less risk. Multiple sclerosis is an autoimmune disease, in which the body’s own immune system attacks the nervous system. Recent research has suggested that autoimmune and allergic diseases such as asthma may be related to the development of the immune system during the first few years of life. One current thought (the “hygiene hypothesis”) is that more infections in early life teaches the immune system in such a way that allergic and autoimmune reactions are less likely.
It is unclear why exposure to siblings is protective against multiple sclerosis; it does match other studies which show that acquiring certain infections, such as mono and human herpesvirus 6, later in life increases the risk of multiple sclerosis. Perhaps having younger siblings means that children get these infections before age 6. It is unclear whether exposure to other children besides siblings is protective. More research is needed, including research to confirm the findings of this study.
