All of my guests — even frequent visitors — are greeted by a volley of loud barks from my fifty-pound dog.

Willie’s hello seems more warning than greeting as she stands at my screen, her ears up, her tail at attention. She continues to bark until you are invited in or leave the doorstep. And heaven help you if you bring a backpack. Before it hits the ground, you can bet she’ll have her nose in it.

It’s behavior that baffles our guests. We’ve seen reactions from outrage to bewilderment. But regardless of the happiness of the visitor in question, I give Willie a pat and tell her she’s a good girl, that she’s done a good job.

Because in Willie’s world, unchecked bags and strange visitors are sources of unease.

Willie is my eleven-year-old Belgian Malinois. Before she came into my life, she served as an explosives detection dog for Blackwater. She lived in Iraq with her handler until her retirement and arrival in the United States at age eight.

Three years later, she is unquestionably my best friend. If I had to guess, I’d tell you she’s made the transition to civilian life well. She certainly seems happy, content to spend her autumn years with me in my small apartment, enjoying endless free tennis balls supplied by a clumsy tennis class at a nearby court and a permanent indention on my shabby couch that serves as a bed. Her behavior, though, is still marked by her breeding and her years finding bombs. She’ll check backpacks until she dies.

I knew little until I plunged in headfirst into the world of working dogs by adopting Willie. The largest and most complex dog I’d ever had, a poodle mix, was an easy assignment compared to Willie’s complex calculus of needs and drives.

While you might think her intelligence is a selling point, in reality, I discourage many would-be owners from adopting one of these dogs. Many dogs with the drive and intelligence to become working animals wind up in shelters, victims of overenthusiastic owners who don’t understand the breed before adopting a dog. They soon become overwhelmed with its intense behavior and dump it. Adopting a working-breed dog is a huge commitment and shouldn’t be taken lightly.

It’s important to know what you are getting.

I didn’t know when I first brought her home. Even though Willie understood me just fine, I had to spend time learning her language.

I didn’t know how to speak to her (kindly but firmly), how to reward her (with a game of fetch, not with treats) or what behaviors were a result of her training and needed to be encouraged (usually the strange ones, like checking backpacks). She requires hours of exercise every week. And she’s really expensive, a strain on my slender college budget.

Life with her isn’t perfect. But despite all of the unusual things about my dog, I’m happier than I’ve ever been. She brings me out of my comfort zone. She vets my visitors. I don’t know who saved who that day three years ago I brought her home.

She’s a good girl. I sleep safely at night knowing Willie stands vigil in my apartment, waiting to bark at anything that might menace me in the night.