(CNN) Wearing his favorite tie, Dr. A. William Frankland settles into his beige armchair and recounts a life story Hollywood producers could only dream of.

He was born as a twin in 1912; began medical school in the 1930s; held a military post in Singapore during WWII that resulted in being held as a POW for more than three years; returned to England after the war and studied under Alexander Fleming, the man who discovered penicillin; became an allergist and developed a pollen count system to help people understand what triggered their allergic reactions.

All of that happened by the 1950s, and in honor, the Allergy Clinic at St. Mary's Hospital in London was named after him.

At that point, he had more than a half-century of his career to go.

"So often, people say, 'How is that you've lived so long?' " he said. "And I say, 'That's just luck, nothing else.' "

At 106 (and 1/2, he'll remind you), Frankland still occasionally consults with patients and contributes articles to journal publications. He loves reading medical journals and keeping up with the field he helped pioneer.

"[This] has been quite a unique experience," Watkins said, "and one that I think very few people will have the privilege to do, even more so, with Dr. Frankland, of this remarkable long life, in which he's seen so many changes, so many challenges, and has been through so many really quite remarkable experiences, which most of us can't even comprehend."

Beyond longevity, what sets Frankland apart is the sharpness of his mind.

He's writing a paper now about how penicillin was discovered, based on his time with Fleming.

William Frankland at his wedding, before he went off to war.

He acknowledges the sadness, as well as the highlights, he has experienced over the years, choosing to swallow the negative memories and fear he experienced and focus on happiness.

Memories from his life remain vivid. He even says he remembers his third birthday, when he indulged in too much cake and ended up sick.

"I'm so interested in not only the present and the future but particularly, I suppose, in the past," Frankland said. "And it's nice looking back on some of the enjoyable trips that I did."

A pioneer in allergies

In 1953, at St. Mary's Hospital in London, Frankland popularized the pollen count, now used worldwide to help doctors and patients understand allergy triggers. He ran tests on himself to make his discoveries.

"I caused acute severe anaphylaxis in myself from an insect," he explained. "Nowadays, you wouldn't be allowed to do such an experiment. But to me, I was determined to find out [how] I reacted to mosquitoes, fleas or whatever insect bites me."

Frankland with his microscope.

His institute focused on hay fever, and his research soon linked hay fever symptoms to pollen, changing everyone's understanding of the condition.

That drive to contribute to his ever-expanding field is still here. Since he turned 100, he has written several articles for publication and isn't quite done yet.

"I wrote four articles from the age of 100 to 105," he said. "Two of them are solos; the others were multiple authors. But now, of course, I'm 106, I'm going to write one more, and it's almost finished."

Keeping your mind strong

A decline in mental abilities over time is an aspect of aging feared by most. But that doesn't mean it has to be inevitable. Keeping the mind active and engaged late in life can actually help generate new brain cells and neural connections.

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"When you get old, [there are] some of the things you can't do," Frankland said from his armchair. "I'm too old to go on runs and keep fit in that sort of way. But I certainly keep my brain going all the time. And I read a lot of scientific journals and things which come to me monthly, and some -- even the British Medical Journal -- once a week."

In 2015, at the age of 103, Frankland received the Order of the British Empire for his significant contributions to the field of allergy research. So, how has he done it all these years?

"I think what helps me is that if you lead a sensible life," he said, "and don't smoke and don't overeat, and do a bit of exercise. Be energetic in whatever it is socially and psychologically and emotionally [that] you're doing. Take these things all in your stride."