Pratique du Shadowing: Introduction to the immune system - Apprendre l'anglais à l'oral avec la vidéo
Création de la leçon...
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Despite being surrounded by harmful organisms, toxins, and the threat of our own cells turning into tumor cells,
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humans manage to survive, thanks largely to our immune system.
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The immune system is made up of organs, tissues, cells, and molecules that all work together to generate an immune response that protects us from microorganisms,
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removes toxins, and destroys tumor cells.
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Hopefully, though, not all at once.
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The immune response can identify a threat, mount an attack, eliminate a pathogen, and develop mechanisms to remember the offender in case you encounter it again,
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all within 10 days.
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In some cases, like if the pathogen is particularly stubborn, or if the immune system starts attacking something it shouldn't, like your own tissue, it can last much longer,
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for months to years, and that leads to chronic inflammation.
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Your immune system is like the military, with two main branches, the innate immune response and the adaptive immune response.
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The innate immune response includes cells that are nonspecific, meaning that although they distinguish an invader from a human cell, they don't distinguish one invader from another invader.
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The innate response is also feverishly fast, working within minutes to hours.
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Get it?
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Feverishly?
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That's because it's responsible for causing fevers.
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The trade-off for that speed is that there's no memory associated with innate responses.
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In other words, the innate response will respond to the same pathogen in the exact same way, no matter how many times it sees the pathogen.
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The innate immune response includes things that you might not even think of as being part of the immune system.
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Things like chemical barriers, like lysozymes in the tears, and a low pH in the stomach, as well as physical barriers like the epithelium in the skin
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and gut and the cilia that line the airways to keep invaders out.
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In contrast, the adaptive immune response is highly specific for each invader.
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The cells of the adaptive immune response have receptors that differentiate one pathogen from another by their unique parts, called antigens.
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Adaptive immunity is also diverse, meaning it can recognize almost an infinite number of specific antigens and mount a specific response against each of them.
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The trade-off is that the adaptive response relies on cells being primed
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or activated so they can fully differentiate into the right kind of fighter to kill that pathogen, and that can take a few weeks.
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But the great advantage of the adaptive immune response is immunologic memory.
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The cells that are activated in the adaptive immune response undergo clonal expansion, which means that they massively proliferate.
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And each time the adaptive cells see that same pathogen, they massively proliferate again, resulting in a stronger and faster response each time that pathogen comes around.
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Once that pathogen is destroyed, most of the clonally expanded cells die off, and that's called clonal deletion.
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But some of the clonally expanded cells live on as memory cells, and they're ready to expand once more if the pathogen ever resurfaces.
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Now it's time to meet the soldiers, which are the white blood cells or leukocytes.
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Hematopoiesis is the process of forming white blood cells, as well as red blood cells and platelets, and it primarily takes place in the bone marrow.
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Hematopoiesis starts with a multipotent hematopoietic stem cell, which can develop into various cell types.
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Its future is undecided.
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Some become myeloid progenitor cells, whereas others become lymphoid progenitor cells.
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The myeloid progenitor cells develop into myeloid cells which include neutrophils,
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eosinophils, basophils, mast cells, dendritic cells, macrophages, and monocytes,
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all of which are part of the innate immune response
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and can be found in the blood as well as in the tissues.
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The neutrophils, eosinophils, and basophils are considered granulocytes because they contain granules in their cytoplasm.
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And neutrophils, in particular, are also referred to as polymorphonuclear cells, or PMNs, because their nuclei contain multiple lobes instead of being round.
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During an immune response, the bone marrow produces lots of cells, many of which are neutrophils.
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Neutrophils use a process called phagocytosis.
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That's where they get near a pathogen and reach around it with their cytoplasm to swallow it whole, so that it ends up in a phagosome.
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From there, the neutrophils can destroy the pathogen using two methods.
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They can use their cytoplasmic granules or oxidative burst.
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First, the cytoplasmic granules fuse with a phagosome to form the phagolysisome.
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The granules contain molecules that lower the pH of the phagolysisome, making it very acidic, and that kills about 2% of the pathogens.
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Now, the neutrophil doesn't stop there.
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It keeps swallowing up more and more pathogens until it's full of pathogens, and at that point, it unleashes the oxidative burst.
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During an oxidative burst, the neutrophil produces lots of highly reactive oxygen species, like hydrogen peroxide.
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These molecules start to destroy nearby proteins and nucleic acids within the phagolysisomes, which are the components of the pathogen that has been ingested.
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The net result is that the pathogen is eliminated.
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Now, in comparison to neutrophils, eosinophils and basophils are far less common.
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They both contain granules that contain histamine and other pro-inflammatory molecules.
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Eosinophils stain pink with the dye eosin, which is where they get their name.
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They are phagocytic cells, even though it's not their primary mechanism of attack.
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They are best known for fighting large and unwieldy helminthic parasites, or worms, by releasing molecules that can poke holes in the outer layer of helminths.
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These cells are also involved in allergic reactions such as atopic dermatitis and allergic rhinitis, also known as hay fever.
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When involved in allergic reactions, eosinophils degranulate, meaning they release various enzymes and proteins within their granules,
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and this causes an inflammatory reaction.
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Next, you have basophils, and they stain blue with the dihemotoxylin
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and unlike neutrophils basophils are non-phagocytic on the flip side they have granules
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that contain histamine
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and other pro-inflammatory molecules therefore they are important in initiating allergic responses finally there are the mast cells
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which live in tissues not in the blood
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and they're very similar to basophils they are also non-phagocytic and are are involved in allergic responses.
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Next up are the monocytes, macrophages, and dendritic cells, which are also phagocytic cells.
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They gobble up pathogens, present antigens, and release cytokines, which are tiny molecules that attract other immune cells to the area.
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Monocytes only circulate in the blood.
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Some monocytes migrate into tissues and differentiate into macrophages, which remain in tissues and aren't found in the blood.
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Dendritic cells are the prototypical antigen-presenting cell.
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Dendritic cells are usually found in sites that are in contact with most external antigens, like the skin epithelium or the gastrointestinal mucosa.
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When dendritic cells are young and immature, they're excellent at phagocytosis, constantly eating large amounts of protein found in the interstitial fluid.
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But when a dendritic cell phagocytosis is a pathogen, it's a life-changing, coming-of-age moment.
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mature dendritic cells will destroy the pathogen and break up its protein into short amino acid chains.
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Dendritic cells will then move through the lymph to the nearest lymph node and they'll perform an antigen presentation, which is where they present those amino acid chains,
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which are antigens, to T cells.
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Antigen presentation is what connects the innate and adaptive immune systems.
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Antigen presentation is something that can be done by dendritic cells, macrophages, as well as monocytes,
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which is why all of these cells are referred to as antigen-presenting cells.
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Dendritic cells are the best at this process because they are the only cells that live where pathogens enter, through the epithelial like skin, gut, and airways.
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And they are the only cells that can traffic from these tissues to lymph nodes, where T cells circulate.
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Now, only T cells with a receptor that can bind to the specific shape of the antigen will be activated, and that's called priming.
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It's similar to how a lock will only snap open when a key with a very specific shape goes in.
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However, T-cells can only see their antigen if it is presented to them on a silver platter, and on a molecular level,
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that platter is the major histocompatibility complex, or MHC for short.
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So the antigen-presenting cell will load the antigen on an MHC molecule and display it to T-cells, and when the right T-cell comes along, it binds.
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The final group of blood cells, the lymphocytes, includes B cells, T cells, and natural killer cells.
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B and T cells make up the adaptive immune response, while natural killer cells are part of the innate immune system.
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B cells and natural killer cells complete their development where they started,
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in the bone marrow, whereas some lymphoid progenitor cells migrate to the thymus where they develop into T cells.
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All of the lymphocytes are able to travel in and out of tissue and the bloodstream.
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Natural killer cells are large lymphocytes with granules, and they target cells infected with intracellular organisms,
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like viruses, as well as cells that pose a threat, like cancer cells.
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Natural killer cells kill their target cells by releasing cytotoxic granules.
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These granules contain molecules that punch holes in the target cell membrane by binding directly to the phospholipids and creating pores,
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and release some molecules that get inside the cell and cause target cells to undergo apoptosis, which is a type of programmed cell death.
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B cells, like T cells, also have a receptor on their surface that allows them to only bind to an antigen that has a very specific shape.
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The main difference is that B cells do not need antigens to be presented to them on an MHC molecule.
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They can simply bind to an antigen directly.
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When a B cell binds to a protein antigen that's on the surface of a pathogen, it's capable of internalizing that antigen, degrading it, and presenting it to T cells.
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So technically, they're also antigen-presenting cells as well.
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Like other antigen-presenting cells, the B cell loads the antigen onto an MHC molecule called MHC2 and displays it to T cells.
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When a T cell gets activated, it helps the B cell mature into a plasma cell, and a plasma cell can secrete lots and lots of antibodies.
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Typically, it takes a few weeks for antibody levels to peak.
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The antibodies, or immunoglobulins, have the exact same antigen specificity as the B cell they came from.
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Antibodies are just the B cell receptor in a secreted form, so they can circulate in the plasma, which is the noncellular part of blood,
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attaching to pathogens and tagging them for destruction.
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Because antibodies aren't bound to cells and float freely in the blood, this is considered humoral immunity, a throwback to the term humors,
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which refers to body fluids.
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Now, the final type of lymphoid cell is the T cell, and it's in charge of cell-mediated immunity.
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T cells are antigen-specific, but they cannot secrete their antigen receptor.
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A naive T cell can be activated
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or primed to allow it to turn into a mature T cell by any of the antigen-presenting cells, but most often it's done by a dendritic cell.
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Now, there are two main types of T-cells, CD4 T-cells and CD8 T-cells, where CD stands for cluster of differentiation.
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There are hundreds of CD markers in the immune system, and these CD markers are useful in telling different cells apart.
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For example, all T-cells are CD3 positive because CD3 is part of the T-cell antigen receptor.
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So CD4 positive T cells are actually CD3 positive, CD4 positive.
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And these cells are called helper cells because they're like generals on the battlefield.
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They secrete cytokines that help coordinate the efforts of macrophages and B cells.
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Helper T cells can only see their antigen if it's presented on an MHC2 molecule.
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CD8 positive T cells are CD3 positive, CD8 positive, and they're called cytotoxic T cells because they kill target cells,
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really similarly to how natural killer cells do it, with one major difference.
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CD8 positive T cells only kill cells that present a specific antigen on an MHC1 molecule, which is structurally similar to the MHC2 molecule,
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whereas natural killer cells aren't nearly as specific in who they kill.
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So, now let's go through a complete immune response with a bacterial pathogen in the lungs.
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To start, the bacteria will have to get breathed in, slip by your nose hairs, past the cilia in the airways,
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and will then have to penetrate past the epithelium layer of the lungs.
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Once it's in the lung tissue, the bacteria will start to divide and might encounter a resident macrophage in the lung tissue, which will ingest the bacteria and start releasing cytokines.
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Those cytokines start the inflammatory process by making blood vessels leaky and attracting nearby eosinophils,
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basophils, and mast cells, which release their own cytokines and granules, amplifying the inflammation.
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Neutrophils from the blood, as well as fresh new ones from the bone marrow, dive into the tissue and join the battle.
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If the pathogen was a virus, natural killer cells would help destroy the infected cells at this point.
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This is all part of the innate immune response.
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Around this point in the infection, immature dendritic cells residing under the epithelium digest the pathogens
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and move from the lung tissue over to a nearby lymph
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node where they present the processed antigen on an MHC2 protein to a naive T cell.
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The dendritic cell, which is part of the innate immune response, bridges the innate and the adaptive immune responses when it presents the antigen to the T cell,
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which is part of the adaptive immune response.
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Sometimes, if the infection is spreading, bacteria might find its way to a lymph node without the help of a dendritic cell.
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In this case, B cells, part of the adaptive immune response,
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might directly phagocytose the bacteria and present it to a naive CD4 positive T cell.
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Either way, if the antigen is the right fit for the T cell, the T cell will begin to differentiate and undergo clonal expansion.
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Differentiated CD4 positive T cells will release cytokines that will induce B cells to differentiate into plasma cells,
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which secrete antibodies that will go into the lymph and then into the bloodstream.
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The antibodies will tag pathogens, making it easier for the phagocytes to eat them.
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Some cytokines will activate macrophages to kill bacteria that have been phagocytized, but cannot be killed by the macrophage alone unless it gets help from its friends,
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the T-cells, of course.
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If the pathogen was a virus living and replicating in the cytoplasm of the infected cells,
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the CD8-positive T-cells would kill any infected cells that express the viral antigen on an MHC1.
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Over time, as the invading pathogen dies off, most of the B and T cells die of neglect, but a few turn into memory B cells and memory T cells,
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which linger for years in case they're needed in the future.
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All right, as a quick recap, the immune system has an innate and an adaptive response.
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The innate immune response is immediate but nonspecific and lacks memory.
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Whereas the adaptive immune response is highly specific and remembers everything, but it takes several days to get started and almost two weeks to peak.
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Helping current and future clinicians focus, learn, retain, and thrive.
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Learn more.
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Thank you.
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Vocabulaire et conseils d’expression pour cette leçon
Cette vidéo contient 159 phrases et 2581 mots à répéter en shadowing. La partie parlée dure 16:26. Le locuteur parle à un rythme naturel d’environ 157 mots par minute, proche d’une conversation courante. Seuls 70 % des mots font partie des 3 000 mots les plus courants en anglais, le vocabulaire est donc exigeant.
Vocabulaire clé de cette vidéo
15 mots de la vidéo à retenir, avec leur prononciation et leur sens :
| Mot | Prononciation | Sens |
|---|---|---|
| immune adjectif | /ɪˈmjuːn/ | immun |
| molecule nom | /ˈmɑ.lɪ.kjul/ | molécule |
| adaptive adjectif | /əˈdæp.tɪv/ | adaptatif |
| tissue nom | /ˈtɪʃ.(j)u/ | tissu |
| killer nom | /ˈkɪlɚ/ | mortel, fatal |
| receptor nom | /ɹɪˈsɛp.tə/ | récepteur |
| activate verbe | /ˈæktɪˌveɪt/ | actionner, activer |
| lung nom | /ˈlʌŋ/ | poumon |
| allergic adjectif | /əˈlɝ.d͡ʒɪk/ | allergique |
| differentiate verbe | /dɪf.əˈɹɛn.ʃi.eɪt/ | distinguer |
| bone nom | /boʊn/ | os |
| whereas | /(h)wɛɹˈæz/ | tandis que, alors que |
| plasma nom | /ˈplæzmə/ | plasma |
| inflammatory adjectif | /ɪnˈflæmət(ə)ɹi/ | inflammatoire |
| marrow nom | /ˈmæɹəʊ/ | moelle, moelle osseuse |
Les verbes à particule que vous entendrez
| Mot | Prononciation | Sens |
|---|---|---|
| break up verbe | rompre, couper | |
| come along verbe | accompagner, venir | |
| go through verbe | traverser | |
| make up verbe | /ˌmeɪk ˈʌp/ | constituer, composer |
Prononciation à surveiller
Le locuteur utilise 14 contractions et formes réduites, comme they're, aren't, doesn't. Prononcez-les sous leur forme courte, telles que vous les entendez.
- Les sons « th »: thrive /θɹajv/, pathogen /ˈpæθəd͡ʒn̩/, epithelium /ˌɛpɪˈθiːlɪəm/
- Les sons « sh » et « zh »: tissue /ˈtɪʃ.(j)u/, differentiate /dɪf.əˈɹɛn.ʃi.eɪt/, presentation /ˌpɹɛzn̩ˈteɪʃn̩/, infection /ɪnˈfɛkʃən/, distinguish /dɪˈstɪŋ.ɡwɪʃ/
- Mots longs — placez bien l’accent: differentiate /dɪf.əˈɹɛn.ʃi.eɪt/, inflammatory /ɪnˈflæmət(ə)ɹi/, immunity /ɪˈmjuːnəti/, mechanism /ˈmɛk.ə.nɪ.zəm/, eliminate /ɪˈlɪm.ɪ.neɪt/
Comment s’entraîner avec cette vidéo
- Écoutez la vidéo en entier une fois sans parler et notez les mots que vous ne connaissez pas.
- Commencez à la vitesse 0,75×, répétez phrase par phrase, puis revenez à la vitesse normale quand cela devient facile.
- Enregistrez-vous et comparez avec l’original, en faisant attention à des mots comme immune, molecule, adaptive.
La grammaire de cette vidéo
Les structures que le locuteur utilise le plus, avec les mots exacts de la vidéo :
| Structure | Dans la vidéo |
|---|---|
| Voix passive be + participe passé — l’accent est mis sur ce qui arrive, pas sur qui le fait | being surrounded · is made · being primed |
| Propositions relatives who / which + proposition — une précision sur une personne ou une chose | expansion, which means · soldiers, which are · cell, which can |
| Present perfect have/has + participe passé — une action passée qui compte encore maintenant | has been ingested · have been phagocytized |
Qu'est-ce que la technique du Shadowing ?
Le Shadowing est une technique d'apprentissage des langues fondée sur la science, développée à l'origine pour la formation des interprètes professionnels. Le principe est simple mais puissant : vous écoutez de l'anglais natif et le répétez immédiatement à voix haute — comme une ombre suivant le locuteur avec un décalage de 1 à 2 secondes. Les recherches montrent une amélioration significative de la précision de la prononciation, de l'intonation, du rythme, des liaisons, de la compréhension orale et de la fluidité.