Animal allergies are most often caused by proteins found in dander, saliva, blood and urine. In addition, animals may pick up pollen and mold in their fur, which can trigger reactions in those with allergic rhinitis.

Animal allergies are frequently triggered by proteins from cats and dogs. Guinea pigs, gerbils, rabbits, birds and other popular pets can also cause animal allergies. Symptoms can also arise from exposure to farm animals (e.g., horses, cattle, sheep) or proteins shed by rats and mice.

The classroom can be another source of animal allergens, because many teachers keep animals in the classroom. In addition, dander is very sticky and children with pets can bring it with them to school on their clothing.

For years, experts believed that children exposed to animals early in life were more likely to develop allergies as their bodies became sensitized to animal allergens. Parents with a history of allergies were counseled to keep their young children away from animals.

However, an increasing body of evidence suggests that this theory may be too simple to be accurate. It now appears that there may actually be a benefit to at least some exposure to animals early in life.

For example, a study by the National Institute of Allergy and Infectious Diseases found that children exposed to low to moderate amounts of cat allergens were more likely to develop allergies. However, children exposed to higher levels of cat allergens were found to have reduced incidences of allergies and a lower rate of asthma.

Other studies have found that children raised on farms and exposed to many types of animals have lower rates of animal allergies. And several researchers have found that children who lived with pets before age 7 are much less likely to develop allergies or eczema during their lifetimes.

For now, most experts agree that parents should take a cautious approach to applying this research to their homes. Until more is known, pet choices should not be based on a desire to prevent allergies, unless there is a family history of animal allergies.

If a child is diagnosed as having an animal allergy, parents should be careful in choosing pets. Some animals present little allergy threat to most people. These include fish, turtles, lizards and snakes.

Young animals, such as puppies and kittens, also are less likely to trigger reactions because they do not have old, dead skin to shed yet. Their smaller size also plays a factor. However, these animals will produce more allergens as they grow.