sex and not dying. that's what biologyis all about. and while the sex part is,i'll grant you, a little bit sexier, not dying isalso really fantastic... something that i, personally,like to do every single day. i, personally, like to notdie in all sorts of ways. like, i don't jump out of planes,i don't go into active combat zones, i don't do heroin, but i can,however, spend time wallowing
in filth with my cutebacon-producing friends here and not have to worry about dying. because, somehow,my body can handle a lot of little devils on my hands,in my air, in my food, little things thatliterally want to kill me. there are more potentialhuman killers in this pig pen than there are in allof the world's prisons, but i don't have to worry aboutit because of the elite team of microscopic assassinsthat live inside my body.
my immune system. ahh! that was reallyclose to my hand! you've heard of some of theselittle ninjas, others maybe not, but everyone knows the workthey do by the trail of dead that they leave behind. pus, being the mostdisgusting example. and the work these guysdo is pretty hardcore. they not only identify incomingenemies, they eliminate them, and then they keep files on them,in case their kind ever comes back.
i don't want to freakyou out, but you, and i, are covered in pathogens right now. and you really can'tblame them for wanting to get a piece of your action. your warm, high-energy, nutrient-rich, salty, watery action. your body is a themepark for these guys, and although the majorityof organisms living inside you actually make your life more comfy, there are some less-helpfulviruses and organisms,
from here on outreferred to as pathogens, that will want to turn your bodyinto a factory for their children. so let's avoid that! we have two basic ways of doing it: innate, or non-specific,immunity that responds to all kinds of pathogens the sameway and very quickly, whether your body has seenthat pathogen before or not. and your acquired, or adaptive,immunity which develops more slowly and requiresyour body to learn the wily ways
of the pathogenbefore it defeats it. every animal has an innateimmune system, even sponges! but only vertebrateshave the acquired kind. you were born with yourinnate immune system. and from the second youwriggled your way out of the sterile environment ofyour mom and into this germy, disgusting world, that systemhas been protecting you. the thing about the innateimmune system is that it doesn't care what it's killing.
it doesn't worry aboutwhether it's offing a virus or bacteria or fungus. its job is to just keepthe enemy from getting in, or once it's in,to sneak up behind it and break its neck, ninja style. the first line of defense inkeeping sketchy characters out are the skin and mucous membranes. the skin has so manyexcellent functions, like keeping your organs in,that it's easy to forget
that its primary purposeis to keep things out. it's oily and kind of acidic,and really not easy to penetrate. and i'm about to rockyour world with this, but your digestive tract is alsotechnically the outside of you. remember how our wholebodies are basically just a built around a tube, right? well, the inside of that tubeis exposed to as much weird, grody stuff as theoutside of the tube. so, your body treats the digestivetract like the front lines
of this war, which is one ofthe reasons why your stomach takes no prisoners with thewhole stomach acid situation. in addition to things like skin,we've also got mucous membranes providing another barrier tomicrobes trying to sneak in. mucus membranes line all ofyour internal surfaces that are exposed to the outside like yourlungs and the inside of your nose, as well as some other partsof your body like the inside of your mouth, and youreyelids and your sex organs. mucous membranes unsurprisinglyproduce mucus,
which is a viscous fluid,you've probably heard of it, and it traps microbes andhelps sweep them away. this is why illness is so oftenassociated with such awe-inspiring amounts of goop. your second line of defenseis your inflammatory response. the honchos here are specializedcells in your connective tissue called mast cells that constantlysearch for suspicious objects, usually unknown proteins, andthen release signaling molecules, like histamine when they find them.
histamine makes yourblood vessels more permeable, which allows a whole bunch offluid to flow to the affected area. and that is whatcauses inflammation, but it also brings in acrap-ton of white-blood cells, infection-fighters, to goall balrog on whatever's trying to make its way in. now, this is great if youget a splinter in your toe or a bunch of viruses in your face, but sometimes something gets intoyou that's not actually dangerous
like pollen or dust or,like, a peanut and your innateimmune system triggers an inflammatory response anyway,even though it's not a big deal. this is what wecall an allergic reaction, and you know what those are like with the swelling, redness,mucus production, itching, and occasionally alittle bit of death. so that is why we takeantihistamines to suppress the histamine trigger so ourimmune systems stop freaking out
about nothing, also,that is why you should always tell people when thereare peanuts in your cookies. most of the immune systemactivity that happens inside your body's fortress is doneby white blood cells, or leukocytes. leukocytes are awesomefor a lot of reasons, but one reason is they'vegot full vip access to anywhere in the body that they wantto go, with the exception of the central nervous system:the brain and spinal cord, which are, for obvious reasons,super high security areas.
leukocytes can movethrough the circulatory system and when they get to aplace where they're needed, they can basically send asignal to ask the capillary to open a gap between it's cellsand then it oozes through that gap to the site of the infection,this is called, get ready for it, diapedesis. from the greek for"oozing through." there are lot of differentkinds of leukocytes, like different branches ofyour own personal microscopic army.
the kind specific to the innateimmune system are phagocytes, more greek, this timephago, meaning eating. and they're just any cellsthat ingest microorganisms through the processof phagocytosis. phagocytes are pretty cool. they can literally chasedown the invading cells, grab them and thencompletely engulf them. and some, like thesuper-abundant neutrophils, move around the bloodstream,and can quickly get
to where the action is. once a neutrophil killsan invading microbe, they basically justroll over and die. dead neutrophils collect togetherinto what we lovingly call pus. the biggest and baddest of thephagocytes are the macrophages, the "big eaters," whichdon't generally travel a lot, but instead hang out likebodyguards in your various organs. not only do theykill outside invaders, they can also detect whenone of your cells has gone rogue,
like a cancer cell,and kill those, too. and they, unlike the neutrophils,don't die once they've killed a bacterium. they can eat up to100 before they die. big eater! of all the grisly stuff thatgoes on in the never-ending street war that is yourimmune system, some of the most gruesome stuff is doneby a kind of cell called "natural killer cells"which reminds me,
i think it's time forour very first open letter. an open letter, to 1973. dear 1973, you had a lot going on, thevietnam war ending, roe v wade, watergate...it wasa tumultuous time. but part of me wishes thatyou, 1973, had an opportunity to name everything in biologybecause you got one chance to name a new type ofimmune cell, and you named it "the natural killer cell"and i freaking love that.
i look around at today's scriptwith all of it's dendritic cells and macrophages anddieapudeesises and i think, what if we let 1973name all these things, would we have spikeydeath cells, devourerers, and oozing action instead? i don't know...maybe youwould have screwed it up, but i don't think youcould have done any worse than all of this gd greek wehave to deal with all the time. thanks for the endangeredspecies act! - hank
ok! natural killer cells,more than just a great name, also the only phagocyte inthe innate immune system that destroys other human cells. when your cells are healthy,they have a special protein on their surface called mhc i mhc for majorhistocompatability complex. but when your cells areinfected, say with a virus, or when they're cancerous,they stop producing that protein. so the natural killers arealways going around checking up
on each of your cells, and whenit finds one that's not normal, it pulls out it's ak47 and unloads. actually, it just bindswith it and then secretes an enzyme that dissolves itsmembrane, but still. killing. finally, dendritic cellsare a type of phagocyte that hangs out on thesurface of much of your body that comes in contactwith the environment in your nose, on your skin,in your stomach and intestines. they eat up pathogens and thencarry information about them
back to the spleen or lymph nodes,where it passes intelligence about what's going on on the warfront to the acquired immune system. i actually studied dendriticcells in my undergraduate thesis and i kinda fell in love with them. they're lethal...but they're also intelligent, great heroes for anyrobert ludlum novel. to be fair though,macrophages can do this too. the activity of these cellsgive us a chance to transfer from the innate immune systemto the acquired immune system,
which is going to makethings a little more complicated. the acquired system has to learnas much as it can about every pathogen it interacts with,store that information, and then use it to inventdefenses against them. it's your super-elite,double-secret strike force delta. you get to work buildingyour acquired immune system immediately after you're born,harvesting bacteria and other stuff not just good bacteria thatcan help your guts out, but also harmful ones thatyour body learns from
and stores information about. that system keeps an eye outfor any foreign substance: a toxin, a virus, a bacteria,even parts of those things that could be tell-talesigns of a bad guy. we call those signs antigens, a word that comes fromantibody generator. an antigen is anything thatcauses your immune system to id a pathogen and thencreate an antibody against it. now antibodies aren't cells,they're highly specialized
proteins produced by b cellsto recognize and help lay the smack-down on intruders. but antibodies can'tkill invaders themselves. they're just littleproteins after all. the best that they can do bythemselves is just swarm all over the invader, making itharder for it to move, and to excrete toxins, or otherwiseinfiltrate healthy cells. but more often,antibodies serve as "tags," attaching themselves to thescumbags and then releasing
chemical signals tonearby phagocytes, alerting them thatit's dinner time. your acquired immune systemalso has its own type of white blood cells. not phagocytes, which goafter everything that looks a little bit sketchy,but lymphocytes, which go after specific things thatthey already know about. there are two majortypes of lymphocytes: the t cells, which formin your bone marrow
and migrate and maturein the thymus gland, behind your breastbone,and b cells, which originate and maturein the bone marrow. what t and b actually standfor is a long story, but if it helps you to remember: t's mature in the thymus,b's in the bone marrow. we have two different types oflymphocytes because our bodies have two differenttypes of acquired immunity, the cell-mediated response whichis for when the cells are
already infected,and the humoral response, for when the infectionis just in the humors, the body's fluid,not in the cells. first, let's look at thecell-mediated response. this process mainlyinvolves t cells, and there are quite a numberof different types of them. helper t cells havea cute-sounding name, but in a lot of ways they call theshots for the whole immune system. while they can't killpathogens themselves,
they activate and directthe cells that can. if 1973 had named them,they might have been called "admiral t cells" orsomething more awesome. helper t cells get theirinformation from other immune cells that are out cracking skulls. say, for instance, a macrophagefinds a pathogen and destroys it. after the deed has been done,it has the ability to shred up the proteinsfrom an invader, and put a bit of that antigenon its membrane surface.
this is called antigen-presentationbecause the cell is... presenting antigens! a helper t-cell can detect whenthis happens and it comes over to attach itself tothe presented antigen. the two cells talk toeach other chemically. the antigen presenting cell producesa chemical called interleukin 1 which basically tellsthe helper t cell, "uh, boss, i found this guy overhere and then i broke his neck and then i stuck his gutsall over my cell wall."
and the helper t cell gives it alook and then releases a chemical called interleukin 2,which is like a bullhorn, an alarm that tells all thelymphocytes in the area, "there are problems here! we've got a problem overhere in sector 15!" this alarm activates a couple ofdifferent things all at once: first, the helper t cellstarts making copies, tons of copies, of itself. most of those copies differentiateinto effector t cells,
which travel aroundsecreting signaling proteins that stimulate other nearbylymphocytes to take action. most of the rest of thembecome memory t cells. they're the ones that keepa record of the intruder and provide us with futureimmunity against it. and now for the saddeststory of the day. what happens when acell gets infected. so infected that it knowsthat it's a goner, that it, in fact, is beingconverted from a healthy,
useful part of the body,to an evil zombie farm, pumping out viruses or bacteria,suddenly co-opted to help destroyeverything it loves? well, with its last bitof strength, it'll start presenting antigens,not asking to be rescued, but instead askingfor a mercy killing. the cytotoxic t cell has thejob of granting that request. once a cytotoxic t cell gets themessage from the helper t cells that there's aninfection to deal with,
it starts patrolling the area fornormal cells presenting antigens. when it finds one, it latchesonto it and releases enzymes that create holes in the cell'smembrane and eventually breaks down the whole cell,killing the cell and the pathogen in the process. a human cell killinganother human cell. now for the humoral response. the humoral response isdesigned to catch pathogens that are floating aroundin your body that haven't
actually invaded anyof your cells yet. the primary players are b cells, which are constantlypatrolling your bloodstream like cops walking the beatuntil they get a signal from a helper t cellthat something's wrong. b cells are covered inantibodies that can detect and bind to a specific antigen. a single b cell can be coveredin a forest of up to 100,000 antibodies, say, for the virusthat causes the common cold,
and the b cell next to it willhave just as many receptors for a different antigen,for chicken pox or something. when a b cell bumps into apathogen that it recognizes, it attaches to it and startscloning itself like crazy. suddenly there are tons of thatb cell with the same receptor but during the cloning process,the clones differentiate into new versions of the originaljust like the t cells did. most turn intoplasma or effector cells, which use the antibody as ablueprint to creating a crap-ton
of antibodies forthat specific pathogen like 200 antibodies per second! once these antibodies are released,they bind to the pathogens like crazy, marking themfor death until a phagocyte can come along anddo the dirty work. the rest of the cloned b cellsmostly become memory cells, which have the samereceptor and stick around, providing futureimmunity from this invader. and we are now very out of time,but i really love this stuff,
so i didn't wantto gloss over anything. mucus, natural killer cells,macrophages killing things, breaking them up and stickingthem on their cell membranes, effector cells spewing outantibodies, and memory cells, making sure that our immunesystems hold that grudge, all because my absolutefavorite thing to do every single day is not die. if you want to review anythingwe discussed in this episode there's a table ofcontents over there.
if you have anyquestions for us, we'll be down in the commentsor on facebook or twitter. and we'll see you next time.