NARRATOR：

Listen to part of a lecture in a biology class.

FEMALE PROFESSOR：

So that's the overview of the human immune system.

But we have a few minutes left. Any questions? George?

MALE STUDENT：

Yes. You talked about T cells..."naive T cells"...

Can you go over that part again, and also...why do we call'em that, anyway?

FEMALE PROFESSOR：

All right. They're-they're known as T cells because they develop in the thymus...

MALE STUDENT：

The what?

FEMALE PROFESSOR：

Thymus...that's T-H-Y-M-U-S. It's a small organ in the body.

Anyway, that's why we call'em that—they come from the thymus.

And T cells are a part of the body's immune system.

They can recognize and eliminate cells from outside the body that might cause disease.

FEMALE STUDENT：

But why "naive"?

I mean, we might call people naive if they don't have enough experience to know about...the dangers of the world.

But how can you call a cell "naive?"

Well, when this type of immune cell encounters a cell from outside the body...like maybe a bacterium...it interacts with that bacterium and learns to recognize it, so whenever the immune cell runs into that kind of bacterium in the future, it'll attack and kill it.

At that point we call it a "memory T cell" because it's learned to recognize a protein marker that identifies this particular kind of bacterium.

But before it's learned to recognize any particular protein from outside the body, we'd call it "naive." OK?

Yeah, I get it.

There's a lot of biochemistry involved...that we'll get into in the next lecture.

But your question reminds me about a study that some of my colleagues are doing. It relates to caloric restriction.

Caloric? Like calories in the food we eat?

Exactly. We're talking about the sugars, carbohydrates, fats...that our bodies burn to get energy...which we measure in calories.

OK, let's back up a little.

Back in the 1930s, a nutritionist at Cornell University put mice on a severely restricted diet.

He gave each mouse in one group 30 percent less food...or, more precisely, 30 percent fewer calories than the mice in the other group...which ate a normal amount.

And the result?

The underfed mice lived much longer than the normally fed ones.

Wow, does that just go for mice?

Apparently not.

Similar results have come from experiments on other animals-from roundworms to... most recently, rhesus monkeys.

These monkeys-two groups of'em-were given all the vitamins and minerals and other nutrients they needed, except that one group got thirty percent fewer calories.

And now, after thirty years or so-about an average lifetime for a monkey-it's clear that the monkeys that have been on the calorie-restricted diet are doing a lot better than the ones on what we'd consider a normal healthful diet.

Like, in terms of blood pressure and lots of other measures, the calorie-restricted monkeys are much healthier, and they just look and act younger than the monkeys in the "normal-diet" group.

And, as a group, they're living longer.

Interesting,...but what's the connection?

Oh, with the immune system?

Well, it's been shown that the immune system becomes much less effective as animals age-that's true in humans too.

We think those naive T cells just get used up-I mean it's not like the body's always making lots of new ones.

And over the course of a lifetime, as T cells encounter more and more strange bacteria or whatever, the naive T cells get turned into memory T cells.

So, later on in life, there are fewer and fewer of these naive T cells left to deal with any new disease-causing organisms that might attack.

Which means less immunity, and the animal-or person-is more likely to get sick.

But caloric restriction... it kinda shocks the system, and one result is...well, those monkeys on the calorie-restricted diet had lots more naive T cells left than you'd expect in monkeys that old.

The expected drop in naive T cells-apparently, the shock of getting thirty percent fewer calories really slows that down.

And after many years, with so many more naive T cells still in reserve, these monkeys are a lot better at fighting off new infections than normally fed monkeys of the same...advanced age.

And that's why they live longer?

Well, it's gotta be one reason.

This is all pretty complex, though...with lots of details yet to be worked out.

But are results the same for humans?

Hard to say.

A good study would take decades, and it's not easy finding people who'd want to take part.

Would you?

And eat thirty percent less?

That'd be tough.

You bet it would.