Being joined by a baby brother or sister before the age of 6 years can reduce your risk of developing the autoimmune disease multiple sclerosis by up to 88%, according to a study in The Journal of the American Medical Association. Anne-Louise Ponsonby, of the Australian National University, and colleagues suggest that the effect results from “altering childhood infection patterns and related immune responses”.

The study compared 136 adults with multiple sclerosis with 272 age- and sex-matched controls in Tasmania, Australia. Based on questionnaire results obtained between 1999 and 2001, the authors conclude that the longer people in the first 6 years of life were exposed to siblings younger than 2 years, the greater the reduction in their risk of developing multiple sclerosis. Exposure for 1–3 years reduced the risk by 43%; 3–5 years of exposure reduced the risk by 60%; and more than 5 years of exposure reduced the risk by 88%.

Blood samples from the study participants were analysed for the presence of IgG specific for Epstein–Barr virus (EBV), which is a common childhood infection. In individuals without multiple sclerosis, increased exposure to a sibling was associated with a decreased IgG response to EBV. This lends support to the hygiene hypothesis, which states that exposure to infections early in life can influence the types of immune response that are induced later in life — for example, to self-antigens in autoimmune disease.

Patricia O'Looney of the National Multiple Sclerosis Society (United States) welcomed further research in this area but was careful to point out that “you're not at a higher risk for multiple sclerosis just because you don't have a sibling” (HealthDay).