For many, dogs are man’s best friends. But for people with pet allergies, dogs and cats can be their worst enemies.

Still, many individuals whose noses resemble Old Faithful whenever Fido walks into the room are still able to keep pets in their homes, as many dog and cat breeds are hypoallergenic. However, several veterinarians and allergy experts say that there is no such thing a truly hypoallergenic pup or kitty.

“I don’t believe there is truly a hypoallergenic dog,” said Dr. Laurel Fritzen, a veterinarian at Deerfield Animal Hospital. “There are breeds that shed less hair but a lot of times people are allergic to the dander, which is the skin, and all animals shed skin cells.”

The Obamas’ new pooch Bo, a Portuguese water dog, was chosen because of his apparent hypoallergenic nature. Malia Obama, the president’s 10-year-old daughter, suffers from animal allergies, although the severity is not known.

According to Susan Becker, the president of the Portuguese Water Dog Club of Greater Chicagoland, the single-coated nature of the breed is what makes people less allergic to them, as it is undercoat shedding that gives most people problems.

“These dogs have hair instead of fur. Their hair grows much as a human’s does,” Becker said. “We usually advise people with allergies to spend a few hours with our dogs just to see how they react.”

Dr. Sai Nimmagadda, an allergist at Lincoln Park’s Associated Allergists and Asthma Specialists Ltd., advises allergic patients to spend a substantial amount of time with the breed they’re interested in adopting. He recommends that families “borrow” a pet for a full two to three weeks in order to allow sufficient time for a reaction to occur.

Other breeds that share the single-coated quality of Porties are poodles, bichon frises and several terrier breeds. But despite these dogs’ shared characteristic, they still can cause someone’s eyes to burn, nose to run or skin to itch.

“There’s not really such a thing as a hypoallergenic animal,” said Dr. James Sublett, the vice chair of  Arlington Heights-based American Academy of Allergy, Asthma and Immunology’s Indoor Environments Committee. “You can be allergic to any warm-blooded animal.”

That’s because animal fur doesn’t cause allergic reactions, said Sublett, who practices in Louisville, Ky. Dogs with long hair such as cocker spaniels or those with closely-cropped fur such as Labrador retrievers are equally likely to make someone sneeze, and even hairless dogs like the Chinese crested or sphinx cats can produce allergens.

“The allergen is in anything the animal secretes,” Sublett said.

Glands in the skin secrete a substance that keeps pets’ skin and fur smooth, said Fritzen. Allergy-inducing pet dander is created when dead skin is shed, along with fur from the animal. Dogs that shed a lot, such as Labradors or collies, will cause more reaction in allergy sufferers simply because dander is released with the fur, according to Nimmagadda.

“Part of it is you don’t have the shedding that transfers the skin cells and the saliva all over the house,” Fritzen said. “You don’t have the environmental distribution of the antigens.”

Nimmagadda said people who suffer from allergies might be better off adopting a dog that needs to be groomed frequently, because regular brushing and trimming prevents dander from entering the air. He also recommended small-to-medium-sized dogs because of a smaller surface area for the dander to settle.

Some patients may have an allergic reaction to the proteins found in animal saliva and urine; when animals lick themselves, the proteins transfer to the fur and can cause a reaction in humans that is similar to if they were directly licked by the cat or dog.

“Most patients, if a dog comes up and licks them, they’ll break out in hives,” Nimmagadda said. “The more cats lick themselves, the more likely it is to cause a reaction.”

Because cats are natural self-groomers, “all they do is lick themselves so they’re pretty much covered with their own saliva,” Fritzen said. “That’s a big reason why you have such a big cat allergy issue.”

According to Sublett, cat saliva is one of the major sources for allergies, and it’s particularly potent due to its tiny size – about three-hundredths the size of the cross-section of a human hair – and its ability to attach to other particles and be transported through the air.

Nimmagadda said these characteristics make it very difficult to find a truly hypoallergenic cat.

“With cats, all bets are off,” Nimmagadda said.

He is quick to point out that the possibility for allergic patients to own a pet is highly dependent on the type and severity of allergies the person has.

Patients who suffer from allergic rhinitis – hay fever-like symptoms like itchy, watery eyes or runny nose and sneezing – can easily manage their reactions with allergy shots, prescription or over-the-counter antihistamines or intranasal steroids, said Nimmagadda.

Even chronic allergy problems, which Sublett described as “feeling like you have a bad cold all the time,” can be managed with medication.

But the American Academy of Allergy, Asthma and Immunology estimates that 3 million children have a more severe reaction – asthma that is triggered by allergens. Nimmagadda said those people probably should stick to a fish or lizard if they want a pet.

“I think it’s so important for families to understand that there are different levels of impact,” Nimmagadda said. “I would be very cautious about introducing pets into a home with asthma.”

Sublett said he would go a step further and never recommend someone with significant allergy problems to get an indoor pet.

“Medication can be helpful,” he said. “But you’re better off if you can avoid exposure.”

Sublett said people should be cautious when considering a pet, because if the allergies remain uncontrolled, getting rid of an animal can cause additional trauma to the entire family.

Nimmagadda, who describes himself as “progressive” in his approach to allowing allergy-sufferers to have pets, said that he thinks the benefits of having a pet in the home often outweighs the inconvenience of dealing with minor allergies.

“Having pets teaches children a sense of responsibility,” Nimmagadda said. “Physicians should try to work with the family to benefit both sides.”

Sublett said that if patients suspect they might have pet allergies, they should consult an allergist for “relatively painless” prick testing that would immediately determine whether there is an animal sensitivity, or if their allergies are exacerbated by other environmental factors.

And although Nimmagadda also recommended a visit to an allergist before taking on a pet, he is hopeful about the impact the Obamas adoption of a hypoallergenic dog might have for families who had ruled out the possibility before.

“More families with a history of allergies with pets may look into it,” he said.