GRAHAM Rook, a professor of medical microbiology at UCL, has just led a study into the apparent conflict between the need for cleaning and hygiene to keep us free of pathogens, and the need for microbial inputs to set up our immune and metabolic systems.
He says: "Our immune systems evolved to destroy microorganisms that cause infections (pathogens) and to kill our own cells only if they become cancerous.
"But in the modern world our immune systems often attack 'forbidden targets' such as harmless molecules in the air (allergens such as pollen), or our own healthy non-cancerous cells (autoimmune diseases, multiple sclerosis) or our gut contents (inflammatory bowel diseases and food allergies).
"So there's a failure of the control mechanisms that should stop the immune system from attacking these forbidden targets.
"Development of these control mechanisms is largely driven by signals from harmless microorganisms to which we're exposed, including those that colonise our guts, airways and skin.
"Children need exposure to microorganisms from their mother (during delivery and breast-feeding) and from other family members. Also, exposing children to the microorganisms of the natural environment in gardens and farms activates the control mechanisms, probably explaining why 'forbidden target' diseases are less common in traditional agricultural communities.
"Our ancestors made homes with natural materials such as untreated timber, thatch and mud, so most microorganisms in their homes were also derived from the natural environment. But modern homes are built with unnatural materials including biocide-treated timber and plasterboard. Such modern homes, especially if damp and deteriorating, contain unnatural communities of microorganisms that can be toxic.
"Therefore, cleaning the home, which is essential for hygiene and depleting pathogens, also removes potentially toxic microorganisms without reducing necessary contact with the mother or nature.
"Moreover, children don't need to risk death from exposure to pathogens to strengthen their immune systems because live vaccines, in addition to protecting from the target organism, exert non-specific immune system strengthening effects.
"However, we need to minimise the exposure of children to cleaning agents. When cleaning agents are breathed or swallowed they act as signals to the immune system that increase the likelihood of allergic responses to whatever's present at the time of the exposure, such as food, pollen or whatever.
"So, cleaning the home is beneficial and will not increase allergies, if we minimise exposure of children to the cleaning agents, and maintain contacts with microorganisms from mother, family and the natural environment. And vaccines contribute additional training inputs to the immune system."