The findings, presented at a meeting of the American Thoracic Society in Toronto, suggest a mother's stress during pregnancy may have lasting consequences for her child.
"This research adds to a growing body of evidence that links maternal stress such as that precipitated by financial problems or relationship issues to changes in children's developing immune systems, even during pregnancy," Dr. Rosalind Wright of Harvard Medical School in Boston said in a statement.
Wright and colleagues found mothers who were the most distressed during pregnancy were most likely to give birth to infants with higher levels of immunoglobulin E or IgE -- an immune system compound -- even though their mothers had only mild exposure to allergens during pregnancy.
Studies in animals have found that a mother's stress amplifies the effects of allergen exposure on the immune system of the developing offspring. The Harvard team set out to see if they could find the same in humans.
They measured levels of IgE from the umbilical cord blood of 387 newborns in Boston.
Babies whose mothers were the most stressed out -- but who had low exposure to dust mites in the home -- still had high levels of IgE in their cord blood, a finding that suggests that stress increased the immune response to dust exposure.
This was true irrespective of the mother's race, class, education or smoking history.
STRESS AS 'SOCIAL POLLUTANT'
"This further supports the notion that stress can be thought of as a social pollutant that, when 'breathed' into the body, may influence the body's immune response," Wright said in a statement.
The study patterns recent findings in children who have undergone stress by Dr. Andrea Danese of the University of London. Researchers there followed 1,000 people in New Zealand from birth to the age of 32.
They found children who had undergone maltreatment -- such as maternal rejection, harsh discipline and sexual abuse -- had twice the levels of inflammation in their blood even 20 years later.
High levels of inflammatory markers such as C-reactive protein, fibrinogen and immune cells increase a person's risk of heart disease and diabetes.
"Stress in childhood may modify developmental trajectories and have a long-term effect on disease risk," said Danese, who presented his findings last week at a conference in Chicago on how early influences affect health and well-being.
Danese said maltreatment in childhood may impair the ability of glucocorticoids -- hormones that inhibit inflammation -- to respond to stress later in life, which could lead to depression and other psychiatric ills.
He said children who have survived maltreatment should get an early start on preventive care for common adult diseases.