WEBVTT NOTE This file was generated by Descript 00:00:00.750 --> 00:00:04.140 England, 1953 in Cambridge and London. 00:00:04.260 --> 00:00:07.350 Scientists are figuring out the structure of DNA. 00:00:07.800 --> 00:00:12.660 Meanwhile, in Oxford, a researcher named James Gowans is trying to solve 00:00:12.660 --> 00:00:18.060 another biological puzzle, the mystery of the disappearing lymphocytes. 00:00:18.480 --> 00:00:22.770 He knows that these small cells flood into the bloodstream continuously 00:00:22.950 --> 00:00:27.210 from their lymphatic system, but where do they go and what do they do? 00:00:28.245 --> 00:00:32.655 To find out, Gowans adds a radioactive label to some cells 00:00:32.805 --> 00:00:34.545 and injects them into rats. 00:00:35.085 --> 00:00:39.885 He discovers that the cells move from the blood to lymph nodes through lymphatic 00:00:39.885 --> 00:00:41.925 vessels and back to the blood again. 00:00:42.495 --> 00:00:44.295 The cells are not disappearing. 00:00:44.625 --> 00:00:48.705 They are circulating again and again today. 00:00:48.705 --> 00:00:52.515 We know that these lymphocytes are the B and t cells that are a 00:00:52.515 --> 00:00:57.585 vital part of our immune system by continuously moving around the body. 00:00:57.915 --> 00:01:03.225 They learn to recognize infections and help us fight them to understand how. 00:01:03.495 --> 00:01:06.075 Let's follow one T-cell on its journey. 00:01:06.375 --> 00:01:08.835 Right now, it's in the bone marrow beginning. 00:01:08.835 --> 00:01:12.345 Its change from a stem cell to a more specialized cell. 00:01:12.915 --> 00:01:18.375 Next, it travels to the thymus where it matures into one of two types of T-cell. 00:01:18.915 --> 00:01:22.875 This CD eight T-cell has been trained and is ready to work. 00:01:23.310 --> 00:01:28.050 But because it hasn't yet been activated, we call it a naive T-cell. 00:01:28.920 --> 00:01:32.640 It moves to the bloodstream, then squeezes through a special blood 00:01:32.640 --> 00:01:34.740 vessel and enters a lymph node. 00:01:35.340 --> 00:01:39.390 Lymph nodes filter fluid and proteins from surrounding tissues. 00:01:39.930 --> 00:01:46.320 The T-cell scans around looking for pieces of virus or bacterial proteins nearby. 00:01:46.380 --> 00:01:48.330 A virus has infected the skin. 00:01:48.960 --> 00:01:53.880 A dendritic cell grabs an infected cell and travels to the lymph node where 00:01:53.880 --> 00:01:59.460 it presents chopped up pieces of viral protein called peptides to the T-cell. 00:02:00.120 --> 00:02:04.680 Fortunately, this T-cell has a receptor that recognizes one of the peptides. 00:02:05.295 --> 00:02:11.055 Kicking off an immune response, the activated T-cell starts dividing some of 00:02:11.055 --> 00:02:13.755 the new cells become effector T cells. 00:02:13.995 --> 00:02:17.745 Special homing receptors tell them to go to the skin where 00:02:17.745 --> 00:02:19.575 they kill infected cells. 00:02:20.235 --> 00:02:22.515 Others become memory T cells. 00:02:22.785 --> 00:02:24.855 Some hang around the infection site. 00:02:25.405 --> 00:02:27.445 Others keep moving around the body. 00:02:27.925 --> 00:02:32.785 They are all ready to kickstart the immune response if they spot this virus again. 00:02:33.685 --> 00:02:38.575 Which brings us back to the observation that Gowans made that T cells keep 00:02:38.575 --> 00:02:40.525 recirculating around the body. 00:02:41.065 --> 00:02:46.615 Why each T cell expresses a unique receptor that can only recognize a 00:02:46.615 --> 00:02:51.385 very few peptides to increase the chance of a T-cell encountering a 00:02:51.385 --> 00:02:53.485 peptide that matches its receptor. 00:02:53.880 --> 00:02:58.830 It must keep moving and searching if all the T cells with their different 00:02:58.830 --> 00:03:03.270 receptors do this, the immune system has a good chance of detecting 00:03:03.270 --> 00:03:05.340 anything that might cause a problem.