According to a study conducted in the US, children living in cities are more prone to have food allergies (milk, egg and peanuts) than those living in the country. The study revealed that those staying in highly populated urban areas are twice more likely to develop the allergies than their rural counterparts. This could be probably because the children in the countryside have higher immunity. They develop the immunity from the bacteria that are prevalent in nature. In the built-up areas, allergies could be triggered by exposure to pollutants at a young age. It may also be seen that there is a positive co relation between exposure to microbes at an early age and building up a child’s defenses against allergies.
Details of the study on Telegraph.