U.S. Marine Corps veteran Corporal Jeff DeYoung had an emotional reunion today with his best friend from his Afghan deployment - Cena,a black Lab.

Cena returned home from duty after working as a military contract service dog since October 2009 when he and DeYoung first deployed to Afghanistan together.

DeYoung, now 23, and Cena were randomly matched together to work towards protecting Marines from the danger of improvised explosive devices - IEDs. Cena would work 50 yards ahead of DeYoung and was able to detect the scent of the IEDs, DeYoung told ABC affiliate WXYZ.

"When he found the scent, he would lay down and would look at me," said DeYoung, of Muskegon, Mich. "At that time, I knew within six feet of that area there was a bomb."

Because of their intense environment, DeYoung and Cena developed an inseparable bond.

"My main goal was to protect him," DeYoung said. "I remember we would get into firefights and having to cover myself on top of him so he wouldn't get harm. It was all about him."

From left, Cpl. Jeff DeYoung in action with his military dog Cena in Afghanistan, and DeYoung reuniting with his military dog Cena at the Detroit Metropolitan Airport.

Cena is a purebred black Labrador with a nose that can smell over 1,000 different scents, and 300 different explosives, DeYoung said.

DeYoung hadn't seen Cena in four years after they were separated without saying goodbye in April 2010, a tactic the military uses to make the separation between handler and service dog easier. DeYoung went home and Cena stayed on in Afghanistan. However, for DeYoung, that separation hit him hard, he said.

DeYoung made it his mission to locate Cena through those four years that he developed his new life at home, he said. DeYoung now is married and has two kids. However, his thoughts kept going back to his buddy.

Mission K9 Rescue is a non-profit organization that works with the military, along with the American Humane Association, to reunite retired service military dogs with their former handlers.

DeYoung and his wife were connected with Kristen Maurer, president of Mission K9 Rescue, once he found out Cena would be up for adoption. Cena was retired from military service due to serving the average time of six years for contract working dogs. She also had a hip injury after years on the job.

Since the military does not fund transportation for the service dogs, Mission K9 Rescue works with the American Humane Association to provide the necessary transportation to reunite the dog with his new family.

Maurer traveled from her home in Houston, Texas, to Raleigh Durham International Airport on Thursday to pick up Cena. Maurer then drove with Cena to a Detroit rendez-vous with DeYoung on Thursday afternoon and was there to witness the emotional reunion.

"The reunion was beautiful. They always are," Maurer told ABC News.

"Cena slammed her head into his chest once she realized who he was and she was so happy," she said.

Maurer said she and her organization have reunited seven service dogs with old handlers this past year alone. The organization has been in operation since 2008.

When DeYoung and Cena finally had their reunion, four years after they last saw each other, DeYoung thanked his friend for taking care of him and introduced Cena to his new family. DeYoung's two children ran up to their new sister to say hello.

"Beacause of you, I got to have them," DeYoung said to Cena.