You roll up your sleeve. A quick pinch, a small bandage, and you're done.

That's how most of us think about a shot. In and out.

But the needle is only the first half. The second half happens inside you over the next few weeks. Your body has to read the shot and build its own defense. Some bodies build a lot. Others build very little.

As we age, that build tends to slow. So here's a fair question: can what's on your plate change how well your body answers?

A team in Northern Ireland set out to test it. First, though, meet the doctor who gave one lung illness its odd nickname.

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William Osler was the most famous doctor of his day. He helped build Johns Hopkins Hospital in Baltimore. His textbook trained a whole generation of doctors.

In its first edition, in 1892, he called pneumonia the special enemy of old age. Later he wrote something gentler. Pneumonia, he said, might well be called the friend of the aged. It took the old quickly, often without much pain, and spared them a long, slow decline.

Doctors still repeat the phrase. You may have heard it: "the old man's friend."

Osler knew the illness better than almost anyone. In 1919, at 70, a chest infection that began as pneumonia took him too.

A century on, things look very different. There's now a shot aimed at the germ behind much of it. But a shot only works as well as the body that answers it...

A vaccine is a kind of wanted poster. It shows your body a harmless piece of a germ, like a photo of its coat.

Special white cells study that poster. Then they start making antibodies. These are tiny Y-shaped proteins, each built to fit one germ and no other. When the real germ shows up, they grab hold and mark it for cleanup.

More antibodies means a firmer grip. Fewer means a weaker hold.

With age, those cells get slower and fewer. That's one reason doctors push shots harder after 65. It's also why the answer to the same shot can vary so much from one older person to the next.

Researchers have long wondered if food plays a part. In Belfast, they tested it the hard way: a real trial, with real vaccines.

The team, led by Jayne Woodside at Queen's University Belfast, found 83 healthy men and women aged 65 to 85. All of them ate very little fruit and few vegetables. Two portions a day, or less.

Chance split the volunteers into two groups. One group kept eating the same way. The other raised its intake to at least five portions a day, for 16 weeks.

Nobody got a strict menu. People picked the fruit and vegetables they liked. Both groups got a free weekly box from a local store, sized to their goal. A dietitian helped people fit it all in.

Then, at week 12, everyone got two shots. One was for tetanus. The other was for pneumonia.

From the research: Over the 16 weeks, the five-a-day group ate about four and a half more portions a day than before. Blood tests backed that up, with vitamin C and several plant pigments on the rise. Four weeks after the shots, their antibodies to the pneumonia vaccine had climbed about 3.1-fold from the start. In the group that kept eating as before, the rise was about 1.7-fold. Each extra daily portion tracked with a response roughly 18 percent stronger. Published in the American Journal of Clinical Nutrition, 2012.

The gain was clearest in people who had never had that pneumonia shot before.

Here's the part worth noticing. No one took a pill. The group didn't hunt down rare or costly foods, either. They just ate more of the plain produce sold at any grocery store.

The honest answer is that nobody knows for sure.

Fruit and vegetables carry vitamin C and a long list of plant pigments. Lab studies have linked some of them to the cells that make antibodies. The Belfast team thinks the whole mix may help those cells do their job. They didn't prove which part matters, or how.

For your kitchen, that's fine. You don't need to know which key turns the lock to carry the whole ring.

This was a small trial. Eighty-three people is not thousands.

The tetanus shot showed no difference at all between the groups. A larger British trial of 217 older adults added fruit, vegetables, fish and nuts. It also found no change in the answer to a tetanus shot.

The Belfast team measured antibodies in the blood. They did not count who later got sick. A higher count is a good sign, but it isn't the same as fewer infections.

And the pneumonia shot used back then has since been joined by newer kinds. Nobody has run this test on those, or on the flu shot.

So this isn't a promise. It's a nudge, from a real trial, in a direction your doctor already backs.

The trial used the British measure of about 80 grams. That's one apple, banana or pear. Or it's three heaping tablespoons of cooked vegetables. A small glass of juice counts too, but only once a day, no matter how many you drink.

Not everything has to be fresh. A bag of frozen green beans or mixed berries counts the same way. So do canned tomatoes, or canned fruit packed in juice instead of syrup. They keep for weeks, need no chopping, and cost less in winter.

In Belfast, people ate the extra portions for 12 weeks before any needle. Nobody tested a quick cram the night before. Treat this as a habit, not a prep step. Begin now, and it's already in place when your next shot comes due.

Osler called pneumonia the old man's friend because, in his day, there was little else a doctor could offer. Now there's a shot. And there's a small, real sign that your plate may help your body answer it.

About four more handfuls a day. That's roughly what the Belfast group added, all from an ordinary store.

Pick tomorrow's extra portions tonight. Set the fruit bowl out. Then call and book the shot.

The needle does the first half. Your plate may help with the second.