German scientists believe it is important children come into contact with other people while still young so that their immune systems can adjust to dealing with infections.

Writing in the medical journal The Lancet, they say that children from larger families tend to pick up infections from their brothers and sisters.

But youngsters from smaller families are less likely to pick up infections, and as a result may be more likely to develop allergies later in childhood.

For this reason the age that these children are sent to nursery may be an important factor in their later susceptibility to allergic responses.

The German team examined a cross-section of children aged 5-14 years from three towns in eastern Germany, Bitterfeld, Hettstedt, and Zerbst.

Of these, 620 children were from small families - often they were the only child - and 1,630 were from large families.

The parents of the children completed a questionnaire about allergies in the family and about their child's attendance at day nursery.

Allergy tests

Children were tested for allergy to grass pollen

The researchers tested all the children for allergic responses.

They used skin-prick tests and measured antibodies against various allergens, such as grass pollen and house-dust mite.

Among the children from small families, those who first went to nursery at a young age (6-11 months) had fewer allergies, including hayfever and irritated eyes, than the children who first attended at an older age (12-23 months).

By contrast, age of entry to nursery had no effect on atopy in children from large families.

The researchers conclude that conclude that "early infection may protect against allergies in later life".

One of the study authors Dr Joachim Heinrich, of the Instititute of Epidemology, Neuherberg, said infection might impact on the development of the body's white blood cells, or lymphocytes.

He said: "Data from several other studies indicate a stimulation of Th1 lymphocytes by infections in early live that may inhibit the expansion of allergen-specific Th2 lymphocytes which is associated with the development of allergic diseases."

A spokeswoman for the National Asthma Campaign said the condition had also been linked with lack of exposure to infection.