Keeping babies in ultra-clean environments, with no exposure to common infections and away from other infants, during the first year of their lives can trigger leukemia - the most common type of childhood cancer - a study has found.
Researchers from the Institute of Cancer Research in the UK assessed the most comprehensive body of evidence ever collected on acute lymphoblastic leukemia (ALL), a cancer of blood-forming tissues.
The research, published in the journal Nature Reviews Cancer, concludes that the disease is caused through a two-step process of genetic mutation and exposure to infection that means it may be preventable with treatments to stimulate or 'prime' the immune system in infancy.
The first step involves a genetic mutation that occurs before birth in the foetus and predisposes children to leukemia - but only one per cent of children born with this genetic change go on to develop the disease.
The second step is also crucial. The disease is triggered later, in childhood, by exposure to one or more common infections, but primarily in children who experienced 'clean' childhoods in the first year of life, without much interaction with other infants or older children.
Acute lymphoblastic leukemia is particularly prevalent in advanced, affluent societies and is increasing in incidence at around one per cent per year.
Mel Greaves from Institute of Cancer Research suggests that childhood ALL is a paradox of progress in modern societies - with lack of microbial exposure early in life resulting in immune system malfunction.
For the study, Greaves compiled over 30 years of research into childhood leukemia.
He challenged previous reports of possible environmental causes, such as ionising radiation, electricity cables, electromagnetic waves or human-made chemicals - arguing that none are supported by robust evidence as major causes.
Instead, he presented strong evidence for a 'delayed infection' theory for the cause of ALL, in which early infection is beneficial to prime the immune system, but later infection in the absence of earlier priming can trigger leukemia.
Childhood leukemia, in common with type I diabetes, other autoimmune diseases and allergies, might be preventable if a child's immune system is properly 'primed' in the first year of life - potentially sparing children the trauma and life-long consequences of chemotherapy, said Greaves.
Population studies have found that early exposure to infection in infancy such as day care attendance and breast feeding can protect against ALL, most probably by priming the immune system. This suggests that childhood ALL may be preventable.
Greaves is now investigating whether earlier exposure to harmless 'bugs' could prevent leukemia in mice - with the possibility that it could be prevented in children through measures to expose them to common but benign microbes.
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