WEBVTT - How Does the Thymus Work (and What Can Happen When It Doesn’t)? 0:00:01.890 --> 0:00:07.490 Welcome to BrainStuff, a production of iHeart Podcasts. Hey BrainStuff, 0:00:07.590 --> 0:00:11.850 Lauren Vogelbaum here. In the human body, some of our 0:00:11.950 --> 0:00:15.280 organs are in the spotlight. We're pretty attuned to what 0:00:15.320 --> 0:00:18.220 our heart and lungs and skin are doing at any 0:00:18.260 --> 0:00:22.580 given time because their status is easily observable. And when 0:00:22.620 --> 0:00:27.140 something goes wonky with them, it's fairly immediately obvious. Other 0:00:27.250 --> 0:00:30.530 organs are more the working quietly in the background types. 0:00:31.180 --> 0:00:34.420 They're no less important. They're all working together to keep 0:00:34.460 --> 0:00:37.449 you alive and kicking. But when they're working well, they're 0:00:37.650 --> 0:00:42.010 not attracting any attention. The thymus is definitely one of those. 0:00:43.390 --> 0:00:46.030 The thymus is a small gland that sits in the 0:00:46.110 --> 0:00:48.849 center of your upper chest, right above your heart and 0:00:48.900 --> 0:00:52.519 just behind your sternum. It's an organ of your immune system, 0:00:52.780 --> 0:00:57.000 and specifically of your lymphatic system. As far as fluids 0:00:57.040 --> 0:00:59.550 in our body go … Blood gets all the glory. 0:00:59.870 --> 0:01:04.910 It's the spotlight fluid. But alongside your blood circulatory system, 0:01:05.209 --> 0:01:08.649 you have a parallel network of vessels and tissues for 0:01:08.690 --> 0:01:14.270 moving around lymph. Okay, what the stuff is lymph? The 0:01:14.330 --> 0:01:17.610 cells in your body are generally not packed in tightly. 0:01:17.930 --> 0:01:22.040 There's fluid surrounding them called interstitial fluid. This is where 0:01:22.080 --> 0:01:25.399 your blood cells deliver oxygen and nutrients to your tissues, 0:01:25.740 --> 0:01:29.700 and where those tissues deposit their waste materials. Your blood 0:01:29.780 --> 0:01:32.240 picks up most of that, but some of it, and 0:01:32.300 --> 0:01:35.300 some of the interstitial fluid it's floating in, can build up, 0:01:35.760 --> 0:01:40.500 and that's where your lymphatic system kicks in. Lymph capillaries 0:01:40.540 --> 0:01:43.850 will pull in the excess fluid and stuff and transport 0:01:43.890 --> 0:01:46.350 it through a series of hundreds of lymph nodes throughout 0:01:46.390 --> 0:01:51.030 your body to, A, screen and filter it for anything unuseful, 0:01:51.640 --> 0:01:55.640 and B, return anything useful to your bloodstream, or right 0:01:55.690 --> 0:01:59.370 before your blood gets pumped back through your heart. Once 0:01:59.430 --> 0:02:02.450 this fluid and stuff is in your lymphatic system, it's 0:02:02.490 --> 0:02:06.770 called lymph, named after Lympha, the ancient Roman goddess of 0:02:06.830 --> 0:02:10.780 fresh water. The lymphatic system is considered part of your 0:02:10.800 --> 0:02:14.739 immune system because a big category of the unuseful stuff 0:02:14.780 --> 0:02:18.210 it screens for is invaders like microbes that could make 0:02:18.250 --> 0:02:21.740 you sick. The main type of bodily cells hanging out 0:02:21.840 --> 0:02:25.560 in the lymphatic system are lymphocytes, which are types of 0:02:25.650 --> 0:02:30.170 white blood cells. These cells can identify and either destroy 0:02:30.210 --> 0:02:34.269 invaders or mark them for destruction. We humans have a 0:02:34.310 --> 0:02:36.990 few types of lymphocytes, but the ones we're concerned with 0:02:37.050 --> 0:02:41.960 today are called T-cells because they develop or mature or 0:02:42.260 --> 0:02:47.560 receive their professional training in the thymus. T-cells are really 0:02:47.600 --> 0:02:50.970 important to your adaptive immune system, that is, the branch 0:02:51.000 --> 0:02:53.870 of your immune system that learns as it fights invaders. 0:02:54.730 --> 0:02:58.090 T-cells can specialize to serve in a number of different functions. 0:02:58.889 --> 0:03:02.690 Some are specially designed to kill particular cancer cells or 0:03:02.770 --> 0:03:05.959 bodily cells that are infected with a virus or bacteria. 0:03:06.900 --> 0:03:10.000 Some will remember a prior invader and can signal other 0:03:10.020 --> 0:03:13.320 white blood cells to help coordinate future attacks against it. 0:03:14.220 --> 0:03:17.000 And some can even help prevent other white blood cells 0:03:17.040 --> 0:03:21.710 from attacking your body's healthy cells by accident. The thymus 0:03:21.850 --> 0:03:25.829 regulates all of this by producing and releasing specific hormones. 0:03:26.690 --> 0:03:30.269 But it does most of this job during your childhood. Like, 0:03:30.560 --> 0:03:33.000 by the time you hit puberty, the bulk of its 0:03:33.060 --> 0:03:36.840 work is done. The T-cells it's created will last you 0:03:36.880 --> 0:03:39.940 for the rest of your life. But your thymus itself 0:03:40.120 --> 0:03:43.840 will slowly decrease in size and function, its active cells 0:03:43.880 --> 0:03:47.960 being replaced with fatty tissue over the years. Today, let's 0:03:48.020 --> 0:03:51.580 look into this odd and quiet but important organ and 0:03:51.740 --> 0:03:54.440 how it works, plus what can go wrong when it's 0:03:54.520 --> 0:03:59.060 not working as expected. Humans noted the existence of the 0:03:59.120 --> 0:04:03.090 thymus in antiquity. It sits right over your heart. It's 0:04:03.160 --> 0:04:06.310 grayish pink in color and and consists of two sort 0:04:06.330 --> 0:04:09.990 of knobby main lobes, each composed of many smaller lobes 0:04:10.070 --> 0:04:14.870 or lobules, all protected by a thin covering. Each lobule 0:04:14.970 --> 0:04:17.210 is made up of a thicker outer layer and a 0:04:17.330 --> 0:04:21.349 less dense inner layer of cells. Your circulatory system brings 0:04:21.410 --> 0:04:24.680 blood to and from the thymus, and lymphatic vessels carry 0:04:24.740 --> 0:04:28.940 lymph out of it. But we essentially had no idea 0:04:29.000 --> 0:04:33.050 what the thymus does until the 1960s. We knew it 0:04:33.110 --> 0:04:36.070 had something to do with the lymphatic system because people 0:04:36.110 --> 0:04:39.630 had observed lymphocytes in it and saw the lymph vessels 0:04:39.790 --> 0:04:43.250 flow out of it. But it was really mysterious because 0:04:43.360 --> 0:04:46.460 it didn't seem to display any changes during an immune 0:04:46.480 --> 0:04:49.640 system response to illness. And taking it out of an 0:04:49.680 --> 0:04:52.700 adult didn't really seem to affect the function of their 0:04:52.740 --> 0:04:57.810 immune system, which we now know makes sense. That's because 0:04:57.870 --> 0:05:00.310 the human thymus does a lot of its job before 0:05:00.350 --> 0:05:04.419 we're even born. We're still learning the nitty-gritty and how 0:05:04.460 --> 0:05:07.890 it can vary from person to person. Until pretty recently, 0:05:07.910 --> 0:05:10.130 a lot of studies about the thymus have been in mice. 0:05:11.150 --> 0:05:15.210 But basically, the thymus is the training and testing ground 0:05:15.290 --> 0:05:18.800 where T-cells must develop and then prove that they're capable 0:05:18.880 --> 0:05:21.640 of doing their job before they go to work out 0:05:21.680 --> 0:05:25.520 in your lymphatic system. T cells begin life in your 0:05:25.540 --> 0:05:28.400 bone marrow, along with a different kind of lymphocyte called 0:05:28.440 --> 0:05:32.350 B cells. B cells stay and develop in the bone marrow, 0:05:32.630 --> 0:05:37.890 but the proto-T cells, called thymocytes, travel to the thymus. There, 0:05:37.990 --> 0:05:42.700 they undergo a series of developments and tests. First, they 0:05:42.740 --> 0:05:45.440 have to prove that they can check the ID of 0:05:45.560 --> 0:05:48.500 a cell or other molecule that might be a threat. 0:05:49.370 --> 0:05:51.710 And then they have to prove that they won't react 0:05:51.860 --> 0:05:55.339 too strongly to anything that presents an ID, marking it 0:05:55.540 --> 0:05:59.360 as coming from inside your own body. After all, you 0:05:59.420 --> 0:06:02.930 want your immune system to eliminate dangerous invaders, but not 0:06:02.960 --> 0:06:06.150 your own healthy cells. You want them to be competent, 0:06:06.510 --> 0:06:11.470 but also self-tolerant. The vast majority of thymocytes do not 0:06:11.529 --> 0:06:14.380 pass these tests and are killed off during the process, 0:06:14.750 --> 0:06:19.099 like over 95% of them. Yet your body produces enough 0:06:19.120 --> 0:06:22.060 of them, and your thymus screens enough of them, that 0:06:22.100 --> 0:06:24.940 you build up a more or less lifetime supply of 0:06:25.000 --> 0:06:29.440 T-cells early on. These will hang out in your lymphatic system, 0:06:29.730 --> 0:06:34.350 awaiting to ID a threat. The thymus also produces T-cells 0:06:34.430 --> 0:06:37.750 that specialize to prevent your other immune cells from attacking 0:06:37.790 --> 0:06:41.390 healthy tissue, and T-cells that act more like first line 0:06:41.410 --> 0:06:45.540 of defense innate immune cells. But again, most of this 0:06:45.580 --> 0:06:49.390 happens when you're really young. As early as one year 0:06:49.470 --> 0:06:52.270 of age, your body might start going through what's known 0:06:52.350 --> 0:06:55.790 as thymic involution, which is a fancy way of saying 0:06:56.029 --> 0:06:59.810 it shrinks. We are born with enough T-cells to last 0:06:59.880 --> 0:07:03.700 us a lifetime if we have to. The thymus stays 0:07:03.839 --> 0:07:07.200 active until puberty kicks in, and then begins shrinking and 0:07:07.240 --> 0:07:10.540 being replaced with fatty tissue at a rate of around 3% 0:07:10.540 --> 0:07:13.280 a year until we reach middle age, around 40 years 0:07:13.340 --> 0:07:15.920 or so, at which point that rate slows to more 0:07:15.960 --> 0:07:19.580 like 1% a year. One thing we still don't know 0:07:19.800 --> 0:07:23.400 is why. It seems like it would be useful for 0:07:23.460 --> 0:07:27.480 our bodies to continue producing T-cells throughout our lives. And 0:07:27.500 --> 0:07:30.800 a lot of the medical issues associated with aging, like 0:07:31.120 --> 0:07:35.480 increased risk of serious illness from infectious diseases, increased occurrence 0:07:35.500 --> 0:07:39.740 of cancer, and increased occurrence of autoimmune diseases like arthritis, 0:07:40.340 --> 0:07:43.310 they're all linked to our immune systems not functioning as 0:07:43.370 --> 0:07:47.330 well as we age. Yet almost all animals that have 0:07:47.370 --> 0:07:52.570 a thymus go through thymic involution. Some researchers have proposed 0:07:52.650 --> 0:07:56.300 that the biological cost of keeping your thymus running is 0:07:56.380 --> 0:08:00.520 simply too expensive. It is true that activity really drops 0:08:00.660 --> 0:08:05.360 off when our reproductive hormones kick in during puberty. Evolutionarily speaking, 0:08:05.740 --> 0:08:08.820 maybe the body focuses its energy on your immune system 0:08:08.920 --> 0:08:11.920 keeping you alive until you hit a reproductive age, and 0:08:11.980 --> 0:08:15.260 then puts that energy into making you capable of reproducing. 0:08:16.490 --> 0:08:20.370 Other researchers have suggested that turning down the thymus' capacity 0:08:20.470 --> 0:08:23.750 after childhood may have some protective effect that we don't 0:08:23.810 --> 0:08:28.450 understand yet. We really don't know. A lot of science 0:08:28.570 --> 0:08:31.630 is being done to better comprehend what the thymus does 0:08:31.690 --> 0:08:35.380 for us later in life, including whether boosting its capacity 0:08:35.559 --> 0:08:38.780 could help prevent some of those immune system-related health issues 0:08:38.880 --> 0:08:42.490 that we see as we age. What we do know 0:08:42.770 --> 0:08:46.189 is that it's not good when your thymus works strangely 0:08:46.570 --> 0:08:50.050 or doesn't work at all. There's a disorder some babies 0:08:50.110 --> 0:08:53.030 are born with called DeGeorge syndrome, where the thymus is 0:08:53.130 --> 0:08:57.990 underdeveloped or missing altogether. This severely decreases the capacity of 0:08:58.030 --> 0:09:01.700 the infant's immune system and may require a thymus transplant. 0:09:03.030 --> 0:09:06.080 In adults, unexpected changes to the thymus have been linked 0:09:06.140 --> 0:09:10.760 to a few immune-related conditions. Those changes can include hyperactivity 0:09:10.780 --> 0:09:14.239 in the thymus, and also cysts, tumors, and cancers of 0:09:14.280 --> 0:09:18.250 the thymus. Some of the conditions linked to these changes 0:09:18.420 --> 0:09:23.090 include a disorder called hypogammaglobulinemia, which just means that your 0:09:23.110 --> 0:09:26.270 body doesn't produce enough antibodies to fight off germs, meaning 0:09:26.410 --> 0:09:30.449 you'll get sick a lot. There's also pure red cell aplasia, 0:09:30.870 --> 0:09:33.390 in which for some reason your body doesn't produce enough 0:09:33.429 --> 0:09:36.690 red blood cells, often because your immune system is attacking 0:09:36.730 --> 0:09:41.600 your bone marrow, meaning you'll experience anemia. And there's one 0:09:41.620 --> 0:09:44.620 we talked about recently on the show, myasthenia gravis, in 0:09:44.660 --> 0:09:47.930 which your immune system attacks the point of communication between 0:09:47.970 --> 0:09:51.069 your nerves and your muscles, leading to muscle weakness that 0:09:51.150 --> 0:09:54.449 starts around the eyes and can proceed through the face, limbs, 0:09:54.550 --> 0:09:58.270 and body. If you or a loved one experiences any 0:09:58.309 --> 0:10:01.460 of these symptoms, talk to a healthcare professional. They'll be 0:10:01.500 --> 0:10:03.939 able to help you rule out other issues and reach 0:10:03.980 --> 0:10:06.880 a diagnosis, and figure out a treatment plan that works 0:10:06.950 --> 0:10:12.270 for you. Those changes in the thymus, like growths, like tumors, 0:10:12.670 --> 0:10:16.790 aren't always noticeable unto themselves. They're sometimes only found during 0:10:16.830 --> 0:10:21.150 an x-ray or CT scan for something else entirely. But 0:10:21.570 --> 0:10:24.410 because of how the thymus is situated in the upper 0:10:24.460 --> 0:10:28.199 center of the chest, thymus tumors can press against a 0:10:28.360 --> 0:10:32.160 bunch of stuff that doesn't like being pressed. This can 0:10:32.200 --> 0:10:36.339 result in chest pain, shortness of breath, hoarseness of voice, coughing, 0:10:36.580 --> 0:10:40.929 trouble swallowing, and the most immediately seriously, a blocking of 0:10:41.030 --> 0:10:44.050 blood coming down from the head, which can cause swelling 0:10:44.170 --> 0:10:47.809 in the face and neck, and headaches or feeling lightheaded. 0:10:48.790 --> 0:10:51.809 Definitely talk to a healthcare professional if you see or 0:10:51.870 --> 0:10:55.329 experience any of those symptoms. Again, they'll be able to 0:10:55.370 --> 0:10:57.819 help you figure out what you're dealing with and prevent 0:10:57.900 --> 0:11:02.240 any danger from blocked veins, which can become deadly. Different 0:11:02.280 --> 0:11:05.550 types of thymus tumors have different options for treatment. Not 0:11:05.590 --> 0:11:07.510 all of them will spread, but you want to get 0:11:07.550 --> 0:11:10.970 that diagnosis to put you on the right path. Even 0:11:11.150 --> 0:11:14.270 if we don't understand everything about organs like the thymus 0:11:14.390 --> 0:11:17.350 that seem to just putter along in the background, we're 0:11:17.410 --> 0:11:20.670 learning more all the time. What we've assessed just since 0:11:20.710 --> 0:11:24.730 the 1960s has changed our understanding of T-cells and thus 0:11:24.850 --> 0:11:28.630 informed how we think about everything from vaccines to cancer 0:11:28.650 --> 0:11:33.300 therapies to the human microbiome. Here's hoping that future research 0:11:33.340 --> 0:11:40.850 will help us unlock even more about human health. Today's 0:11:40.910 --> 0:11:44.230 episode was written by me, Lauren Vogelbaum. The Brain Stuff 0:11:44.270 --> 0:11:46.430 is a production of iHeart Podcasts and is produced by 0:11:46.470 --> 0:11:49.910 Tyler Klang. For more shows from iHeart Podcasts, visit the 0:11:50.010 --> 0:11:53.189 iHeart Radio app, Apple Podcasts, or wherever you listen to 0:11:53.210 --> 0:11:54.050 your favorite shows.