Babies born by caesarean section have a weaker immune system than those born vaginally, which could explain why they are more likely to develop allergies and asthma, a new study says.

Researchers measured the immune cells in the umbilical cords of 50 babies delivered vaginally and 68 babies delivered by c-section. Each baby had at least one parent with allergies or asthma.

They found that regulatory T cells, which influence the action of effector T cells that are involved in immune system development, did not function as well in the babies born by c-section versus the babies delivered vaginally.

This poor immune system function could, in turn, lead to the development of allergies or asthma later in life.

The research was conducted by a team of scientists from the University of California at San Francisco and presented Tuesday at the American Thoracic Society's 2008 International Conference in Toronto.

At the same conference, other researchers presented evidence that a firstborn child is more likely to develop allergies or asthma due to the presence of a gene variant that is linked to both conditions.

The researchers who studied the delivery methods' role in allergies and asthma speculate that vaginal labour exposes a baby to important bacteria that activate the immune system. They also guessed that the stress of labour itself could trigger hormonal responses that may have an impact on a baby's immune system development.

"These findings are preliminary and further work is needed to explore potential mechanisms for the association between mode of delivery and neonatal immune responses," lead study author, Dr. Ngoc Ly of UC San Francisco, said in a statement.