Six new studies suggest that the development of an infant's immune system is affected by a number of early-life factors, including breastfeeding, that can shape an infant's susceptibility to certain pathogens. Gestational age  -- how long the fetus was in the womb before birth -- and delivery by Cesarean section were two other factors. The studies are being presented at the annual meeting of the American Academy of Allergy, Asthma and Immunology in Houston, which began Friday and ends Tuesday.

Dr. Christine Cole Johnson, one of the researchers for the studies, said that their findings support what is known as the "hygiene hypothesis," or the idea that childhood exposure to germs and certain pathogens help the immune system's development by teaching the body not to overreact to harmless substances and to fight dangerous ones instead.

"For years now, we've always thought that a sterile environment was not good for babies," Johnson said in a release. "The immune system is designed to be exposed to bacteria on a grand scale ... If you minimize these exposures, the immune system won't develop optimally." In addition, she said, “The research is telling us that exposure to a higher and more diverse burden of environmental bacteria and specific patterns of gut bacteria appear to boost the immune system’s protection against allergies and asthma.”

The six separate studies looked at maternal or birth factors to determine whether and how they affect the gut microbiome, or the bacteria in the GI tract, for infants, and what impact those bacteria had on the development of allergies or asthma, as well as the development of T cells -- white blood cells that are essential to a healthy immune system. The maternal and birth factors included the mother's race or ethnicity, gestational age, pre- and postnatal-exposure to tobacco smoke, vaginal versus Cesarean delivery and whether or not a home had pets.

Babies who were breastfed at the age of 1 and 6-months-old had specific and distinct bacteria in their guts that those who were not breastfed did not have, the researchers discovered, and the particular composition of those bacteria in the breastfed children may affect the development of their immune system. Researchers also found that babies breastfed at 1-month-old were less likely to have pet-related allergies, and that children who had nighttime coughing had distinct gut bacteria in the first year of life.

These six studies are not the first to investigate and reinforce the possibility that breastfeeding improves the development of an infant's immune system. A 2014 study in the Journal of Allergy and Clinical Immunology found that babies up to one year of age exposed to bacteria and allergens might have less risk of developing allergies and asthma when they are older. And in 2002, the Henry Ford Hospital produced a study that found that children exposed to dogs and cats during the first year of life had reduced risk of allergies.