Playgroups 'cut leukaemia risk'
Children who attend daycare or playgroups cut their risk of the most common type of childhood leukaemia by around 30 per cent, a study estimates.
Researchers reviewed 14 studies involving nearly 20,000 children, of which 6,000 developed acute lymphoblastic leukaemia (ALL).
It is thought early infections may help the body fight off the disease.
The University of California, Berkeley study will be presented to a leukaemia conference in London.
Leukaemia is the most common cancer found in children in the industrialised world, affecting about one in 2,000 youngsters.
ALL accounts for more than 80 per cent of leukaemia cases among children, and most often occurs in those aged between two and five.
Scientists believe that for most types of childhood leukaemia to develop, there must first be a genetic mutation in the womb, followed by a second trigger - such as an infection - during childhood.
However, it is also thought that contracting some childhood infections - which are often readily spread in environments such as playgroups where children are in close contact with each other - may prime the immune system against leukaemia.
Conversely, if the immune system is not challenged in early life, this is thought to raise the risk of an inappropriate response to subsequent infections, making the development of leukaemia more likely.
The US team found that 12 of the studies suggested some protective effect from social interaction.
Dr Patricia Buffler leader of the research team from the University of California
They found that if several of the weaker studies were weeded out the protective effect could be as high as 40 per cent.
However, research showed that children from large families were less likely to benefit than those with fewer brothers and sisters.
Lead researcher Professor Patricia Buffler said: "Combining the results from these studies together provided us with more confidence that the protective effect is real."
Edward Copisarow, of the charity Children with Leukaemia, said: "These findings are important because this is the first time the results of all the relevant studies have been put together and it clearly shows that there is an effect here.
"This is the kind of research that brings us a step closer to understanding the causes of this complex disease and how we can prevent it."
Professor Jillian Birch, of the charity Cancer Research UK, said many studies had found evidence for a link between infection and childhood leukaemia - but exactly how infection affected a child's risk remained unclear.
"Until we have conclusive evidence on the risk factors for childhood leukaemia and an understanding of a mechanism behind its link with infection, it is too early to make recommendations on how to avoid this relatively rare disease."
Dr Carole Easton, of the charity CLIC Sargent, cautioned against drawing firm conclusions.
She stressed that the theory that the disease was linked to infections was unproven.
"What this study does say is that there is a need for further comprehensive research.
"Until we have conclusive evidence then we cannot say for certain what causes childhood leukaemia."