The study, funded in part by the National Institute of Allergy and Infectious Diseases (NIAID), begins to uncover the immune system processes behind this phenomenon. This work was also supported by the National Institute of Environmental Health Sciences (NIEHS).

For many allergens, such as the house dust mite, the higher the level of exposure, the higher the likelihood of a person producing "allergic" antibodies (called immunglobulin E or IgE antibodies). High allergen levels also increase a person's risk of becoming allergic and developing asthma.

Thomas A. Platts-Mills, M.D., Ph.D., and colleagues at the University of Virginia's Asthma and Allergic Diseases Center have shown that cat exposure is different. The researchers measured the levels of antibodies to cat allergen in 226 children, aged 12 to 14 years, and tested the children for asthma. They also measured the amount of cat allergens in the children's homes and discovered that low-to-moderate amounts of cat allergen seemed to trigger allergy, but high amounts reduced both IgE antibodies and the likelihood of asthma.

"This result alters the advice we give patients," says Dr. Platts-Mills. "I would not recommend that parents get rid of their cat because they are concerned their child might develop asthma. However, high exposure to cat allergen appears to be protective for some children and a risk factor for others. If the child is wheezing and has a positive skin test to cat allergen, then you should get rid of your cat."

The high levels of cat allergen prompted the children's immune systems to predominantly make immunoglobulin G (IgG) and IgG4 antibodies rather than IgE, explains Marshall Plaut, M.D., chief of the allergic mechanisms section at NIAID. "This research sheds more light on the relationship between allergen exposure and asthma. When investigators further understand this process, it might lead to new treatments for asthma."