A summary of some of the recent, occasionally contradictory research on immunity and exposure to "unclean" environments:

* A study published in the Journal of the American Medical Association in August showed that exposing children to two or more pets during their first year of life is likely to help create an immunity to allergies. Researchers at the Henry Ford Hospital in Detroit followed 474 infants for about seven years and found that children exposed to two or more cats or dogs in their first year of life were 75 percent less likely to be sensitive to common allergy triggers at age 6 compared with those with no pets.

* Researchers at Brigham and Women's Hospital in Boston looked at 448 children with a father or mother with asthma. Those who had a cat around the house when they were infants were less likely to experience asthma and wheezing at age 1 than the other children. This apparent protection continued until the end of the study, when the participants turned 5. But some cat-exposed children -- particularly those with asthmatic mothers -- wheezed more than those in cat-free homes. The study was published in The Lancet in September.

* A study that appeared in the American Journal of Respiratory and Critical Care Medicine in September showed that children with cats were less likely to have asthma than those without cats. University of Virginia researcher Matthew Perzanowski and colleagues surveyed more than 3,400 Swedish children, concluding that having a cat provided a protective effect from asthma. Weaker protection was seen for having a dog.

* European researchers vacuumed dust from the beds of 812 children from rural areas of Austria, Germany and Switzerland, then measured the dust for endotoxin. The resulting study, published in the New England Journal of Medicine in September, showed that kids who had the cleanest mattresses had the most hay fever, asthma and allergic reactions.

* Researchers interviewed 18,000 adults from 36 countries, including the United States, Australia and much of Europe. Blood samples were taken to find antibodies that indicate when a person may be at risk for allergies to common triggers such as dust mites, pet dander and pollens. It turned out that allergies and hay fever were less common among people who had many siblings or those who attended day care. However, asthma was found to be more common among children with more than two siblings and those enrolled in day care. The study appeared in the journal Thorax in October.

-- Suz Redfearn