It happened again on the trip home. “Across from her, the same passenger with her dog were also returning to L.A.,” Morris remembers. “My daughter told the flight attendant about her allergies, and they moved my daughter to a seat in the very back row.”

Morris wants to know — and so do other passengers who contact me regularly — who should be moving: the pet or the passenger?

Fortunately, human-animal conflicts on planes appear to be relatively rare. The U.S. Department of Transportation recorded only 22 complaints about pets on aircraft in 2013, and so far this year, it has received 18. Grievances about service animals used by passengers with disabilities are also minuscule — a total of 35 to date, compared with 45 last year, according to the department.

Airlines are concerned about the welfare of passengers who suffer from allergies. For example, Delta Air Lines in 2012 adopted a new policy for passengers with peanut allergies, and, when notified of an allergic passenger, will refrain from serving peanuts and peanut products aboard the flight.

But pet allergies are more complicated. Pets generate sizable revenue for airlines in the form of extra fees and are unlikely to be removed from a flight just because another passenger complains. What’s more, the Transportation Department requires that they allow service animals on flights.

“Carriers should do their best to accommodate other passengers’ concerns by steps like seating passengers with service animals and passengers who are uncomfortable with service animals away from one another,” it says in a 2009 rule on nondiscrimination and air travel.

A closer look at the federal regulation reveals one or two loopholes that could come to the rescue of passengers like Morris’ daughter. First, there’s no requirement that all pets be allowed in the cabin, only service animals, although many airlines allow pets small enough to fit in a carrier under the seat. Airlines must accept service animals in the cabin as long as they don’t “pose a direct threat to the health or safety of others” or cause a “significant” disruption in cabin service. The DOT lists as examples an animal that would display threatening behavior, such as “growling, snarling, lunging at or attempting to bite other persons on the aircraft.”

The small dogs on the Sun Country flight might have qualified as “emotional-support” animals, a type of service animal. Sun Country did not respond to several requests for comment, but its policy on these four-legged passengers is available on its website.

Specific to what it calls “psychiatric-assist and emotional-support animals,” as permitted by DOT rules, the airline requires a letter from a licensed mental health professional or medical doctor specifically treating the passenger’s disability. The letter must also be dated and have been written within a year, and the disorder must be recognized by the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition.

The government draws no distinction between a service animal — which is identified by the presence of harnesses, tags or the credible verbal assurances of a qualified individual with a disability using the animal — and one used for emotional support. But passengers do. An incident in October 2000, in which a 300-pound potbellied Vietnamese pig used by a first-class passenger for emotional support reportedly ran amok on a US Airways flight, drew something of a line. An FAA investigation cleared the pig’s owner and the airline of any wrongdoing, but US Airways reportedly vowed to never allow a pig to fly in the passenger cabin again.

Passengers are understandably skeptical of these special designations. “I’m not allergic myself but am of the opinion that there is a time and place for pets, and people are really pushing those limits,” says Scott Hassel, a San Francisco marketing executive. “I love animals, but they don’t need to be inserted into everyone else’s lives. I see people bringing pets on airplanes, trains, into grocery stores and restaurants. No one has any consideration for those around them anymore. Just another symptom of the ‘me, me, me’ culture we live in.”

Unfortunately for allergy sufferers, it usually doesn’t matter why the animal is in the cabin. Chances are, unless it’s growling, hissing or biting other passengers, it’s staying on board. That means allergy sufferers must take sensible precautions. Calling the airline before your flight to let it know about your allergy is a good first step, although it won’t guarantee a pet-free flight. Carrying an EpiPen or allergy medication is a must, particularly when you’re in an enclosed cabin.

Christopher Elliott maintains a consumer advocate website at elliott.org.