Shadowing Practice: Introduction to the immune system - Learn English Speaking with Video
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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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Vocabulary and speaking notes for this lesson
This video has 159 sentences and 2581 words to shadow. The speech runs for 16:26. The speaker talks at a natural 157 words per minute, close to everyday conversation. Only 70% of the words are among the 3,000 most common in English, so the vocabulary is demanding.
Key vocabulary in this video
15 words from the video worth learning, with pronunciation and meaning:
| Word | Pronunciation | Meaning |
|---|---|---|
| immune adjective | /ɪˈmjuːn/ | Exempt; not subject to. |
| molecule noun | /ˈmɑ.lɪ.kjul/ | The smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by… |
| adaptive adjective | /əˈdæp.tɪv/ | Of, pertaining to, characterized by or showing adaptation; making or made fit or suitable. |
| tissue noun | /ˈtɪʃ.(j)u/ | Thin, woven, gauzelike fabric. |
| killer noun | /ˈkɪlɚ/ | One who or that which kills. |
| bind verb | /ˈbaɪ̯nd/ | To tie; to confine by any ligature. |
| receptor noun | /ɹɪˈsɛp.tə/ | A protein on a cell wall that binds with specific molecules so that they can be absorbed into the cell in order to control certain functions. |
| activate verb | /ˈæktɪˌveɪt/ | To encourage development or induce increased activity; to stimulate. |
| lung noun | /ˈlʌŋ/ | A biological organ of vertebrates that controls breathing and oxygenates the blood. |
| allergic adjective | /əˈlɝ.d͡ʒɪk/ | Of or pertaining to allergy. |
| differentiate verb | /dɪf.əˈɹɛn.ʃi.eɪt/ | To modify so as to create a difference or distinction. |
| bone noun | /boʊn/ | A composite material consisting largely of calcium phosphate and collagen and making up the skeleton of most vertebrates. |
| whereas | /(h)wɛɹˈæz/ | In contrast; whilst on the contrary; although. |
| plasma noun | /ˈplæzmə/ | A state of matter consisting of partially ionized gas and electrons. |
| node noun | /noʊd/ | A knot, knob, protuberance or swelling. |
Phrasal verbs you will hear
| Word | Pronunciation | Meaning |
|---|---|---|
| die off verb | To suffer a series of deaths; to die one by one; to progressively become extinct. (If a group of plants, animals, or people dies off, the entire group dies… | |
| break up verb | To break or separate into pieces. | |
| come along verb | To accompany. | |
| come around verb | /kʌ.məˈɹaʊnd/ | To change one's mind, especially to begin to agree or appreciate what one was reluctant to accept at first. |
| go through verb | To travel from one end of something to the other. | |
| make up verb | /ˌmeɪk ˈʌp/ | To constitute, to compose. |
Pronunciation to watch
The speaker uses 14 contractions and reduced forms, such as they're, aren't, doesn't. Say them the short way, as you hear them.
- The “th” sounds: thrive /θɹajv/, pathogen /ˈpæθəd͡ʒn̩/, epithelium /ˌɛpɪˈθiːlɪəm/
- The “sh” and “zh” sounds: tissue /ˈtɪʃ.(j)u/, differentiate /dɪf.əˈɹɛn.ʃi.eɪt/, presentation /ˌpɹɛzn̩ˈteɪʃn̩/, infection /ɪnˈfɛkʃən/, distinguish /dɪˈstɪŋ.ɡwɪʃ/
- Long words — get the stress right: differentiate /dɪf.əˈɹɛn.ʃi.eɪt/, inflammatory /ɪnˈflæmət(ə)ɹi/, immunity /ɪˈmjuːnəti/, mechanism /ˈmɛk.ə.nɪ.zəm/, eliminate /ɪˈlɪm.ɪ.neɪt/
How to practise with this video
- Listen to the whole video once without speaking and note the words you do not know.
- Start at 0.75× speed, shadow it sentence by sentence, then go back to normal speed once it feels easy.
- Record yourself and compare with the original, paying attention to words like immune, molecule, adaptive.
Grammar in this video
The structures the speaker uses most, with the exact words from the video:
| Structure | In the video |
|---|---|
| Passive voice be + past participle — the focus is on what happens, not who does it | being surrounded · is made · being primed |
| Relative clauses who / which + clause — extra information about a person or thing | expansion, which means · soldiers, which are · cell, which can |
| Present perfect have/has + past participle — a past action that still matters now | has been ingested · have been phagocytized |
What is the Shadowing Technique?
Shadowing is a science-backed language learning technique originally developed for professional interpreter training and popularized by polyglot Dr. Alexander Arguelles. The method is simple but powerful: you listen to native English audio and immediately repeat it out loud — like a shadow following the speaker with just a 1–2 second delay. Unlike passive listening or grammar drills, shadowing forces your brain and mouth muscles to simultaneously process and reproduce real speech patterns. Research shows it significantly improves pronunciation accuracy, intonation, rhythm, connected speech, listening comprehension, and speaking fluency — making it one of the most effective methods for IELTS Speaking preparation and real-world English communication.