OFFICIAL45 Why does the professor mention the thymus?

[00:00.00]NARRATOR: Listen to part of a lecture in a biology class.
[00:04.51]FEMALE PROFESSOR: So that's the overview of the human immune system. [00:07.81]But we have a few minutes left. [00:10.24]Any questions? George?
[00:12.59]MALE STUDENT: Yes. You talked about T cells…“naive T cells”…[00:17.82]Can you go over that part again, [00:19.51]and also…why do we call’em that, anyway?
[00:22.49]FEMALE PROFESSOR: All right. They're-they're known as T cells because they develop in the thymus…
[00:27.51]MALE STUDENT: The what?
[00:28.55]FEMALE PROFESSOR: Thymus…that's T-H-Y-M-U-S. [00:33.80]It's a small organ in the body.[00:35.58] Anyway, that’s why we call'em that —they come from the thymus. [00:39.05]And T cells are a part of the body's immune system. [00:42.40]They can recognize and eliminate cells from outside the body that might cause disease.
[00:47.46]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.
[00:56.50]But how can you call a cell “naive?”
[00:58.64]FEMALE PROFESSOR: Well, when this type of immune cell encounters a cell from outside the body…like maybe a bacterium…
[01:04.09]It interacts with that bacterium and learns to recognize it, [01:08.05]so whenever the immune cell runs into that kind of bacterium in the future, it'll attack and kill it. [01:14.21]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. [01:23.72]But before it's learned to recognize any particular protein from outside the body, we'd call it “naive.” OK?
[01:30.93]FEMALE STUDENT: Yeah, I get it.
[01:32.06]FEMALE PROFESSOR: There's a lot of biochemistry involved…that we'll get into in the next lecture. [01:36.23]But your question reminds me about a study that some of my colleagues are doing. [01:40.00]It relates to caloric restriction.
[01:42.66]FEMALE STUDENT: Caloric? Like calories in the food we eat?
[01:46.21]FEMALE PROFESSOR: Exactly. We’re talking about the sugars, carbohydrates, fats…that our bodies burn to get energy…which we measure in calories.
[01:54.05]OK, let's back up a little.
[01:56.51]Back in the 1930s, a nutritionist at Cornell University put mice on a severely restricted diet. [02:03.25]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. [02:13.69]And the result? The underfed mice lived much longer than the normally fed ones.
[02:19.55]MALE STUDENT: Wow, does that just go for mice?
[02:23.31]FEMALE PROFESSOR: Apparently not.  Similar results have come from experiments on other animals—from roundworms to... most recently, rhesus monkeys.
[02:32.71]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. [02:42.06]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. [02:54.65]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. [03:05.36]And, as a group, they're living longer.
[03:08.03]MALE STUDENT: Interesting,…but what’s the connection?
[03:11.11]FEMALE PROFESSOR: Oh, with the immune system?[03:12.84] Well, it's been shown that the immune system becomes much less effective as animals age—[03:18.06]that’s true in humans too. [03:19.78]We think those naive T cells just get used up—[03:23.49]I mean it's not like the body's always making lots of new ones. [03:26.34] 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. [03:36.48]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.
[03:50.53]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. [04:03.96]The expected drop in naive T cells—apparently, the shock of getting thirty percent fewer calories really slows that down. [04:12.58]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.
[04:24.44]MALE STUDENT: And that's why they live longer?
[04:26.16]FEMALE PROFESSOR: Well, it's gotta be one reason. [04:27.68]This is all pretty complex, though…with lots of details yet to be worked out.
[04:32.17]MALE STUDENT: But are results the same for humans?
[04:34.57]FEMALE PROFESSOR: Hard to say. A good study would take decades, [04:38.26]and it's not easy finding people who'd want to take part. Would you?
[04:42.33]MALE STUDENT: And eat thirty percent less? [04:44.58]That'd be tough.
[04:45.99]FEMALE PROFESSOR: You bet it would.