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- Now that we've touched on all of the major players in the
- specific immune system, what I thought I would do in this
- video is do a summary so it all fits
- together a little bit.
- So the first person or character we got exposed to
- was the B cell, which I always do in blue.
- And what made that interesting is that every B cell has its
- own specific-- or they have membrane bound antibodies, but
- for each B cell, the membrane bound antibodies on each
- specific B cell had its own variable portion.
- So this B cell-- it'll be variable right like that.
- And if I were to draw another B cell right here, I would
- draw the variable portion a little bit different.
- This is why different actual B cells will respond to
- different antigens or different pathogens that have
- entered our system.
- And a B cell gets activated-- let's talk about what happens
- when it gets activated or what needs to happen.
- It needs binding of the pathogen onto one of these
- membrane bound antibodies.
- But that's not all.
- I mean, sometimes that's all you need, but usually you also
- need to be stimulated by a T cell.
- And you might say, where's the helper T cell
- stimulate this guy?
- Well, B cells were also antigen presenting cells so
- he'll suck this guy in, break him out, and present him on an
- MHC II complex.
- Let's say this is an MHC II complex.
- This guy gets cut up, part of him gets presented right here,
- and then an activated helper T cell whose variable portion of
- their T cell receptor is specific to this could come
- along and activate this character.
- I'm not drawing that receptor well, but that right there is
- a helper T cell and that is the B cell.
- Now, once it's activated, it starts differentiating-- and
- it starts cloning itself and it can either turn into
- effector cells-- and this true of B cells or T cells.
- Once they get activated, they keep cloning or they either
- turn into effector cells or memory cells.
- Memory cells stick around a lot longer so that in the
- future you're going to have many more of this
- version of B cells.
- So if you get the same antigen or pathogen in the future, the
- likelihood of it bumping into this type of B cell is going
- to be higher so the response will occur faster.
- The effector B cells produce-- they essentially turn into
- antibody making machines.
- So they'll say, gee, this antibody bonds to this antigen
- that we have in the system now.
- Let me just produce a ton of them.
- So it starts building up all of the cellular machinery and
- it starts producing antibodies like crazy.
- I want to point out one thing that my wife pointed out to me
- when I was-- she overheard me making the last video and
- she's a fellow in hematology and a lot of hematology is
- immunology.
- So I definitely have to defer to her.
- She is the expert on this.
- In the last video, I kind of very hand-wavingly said, B
- cells, once they get activated, if they're the
- effector B cells, they produce antibodies.
- I want to be very clear.
- It is only the effector B cells that produce the
- antibodies and the common term for them-- if someone were to
- walk up to you and say, what cells in the body are
- producing antibodies?
- You wouldn't be wrong if you said effector B cells, but the
- common term that people expect to hear are plasma cells.
- Plasma cells and effector B cells are the same thing, but
- normally when they say, what happens to a B cell when it
- starts producing antibodies?
- They then call it a plasma cell.
- They don't call it a B cell anymore and I want to make
- that very clear because my wife's like, well, I have
- attendings that if they asked me, what cell in the body
- produces antibodies?
- And if you said, B cell, they would say, no, wrong.
- It's a plasma cell-- or if you said effector B cell, they
- wouldn't be happy.
- They wanted to hear plasma cell.
- This is the common term used in immunology and apparently
- rheumatology circles.
- Did I just say my wife is a hematologist?
- No, no.
- She's studying rheumatology.
- I get confused with all the tologies sometimes.
- Anyway, that's what the B cells do.
- And these antibodies can then go attach things and mess up
- viruses and antigen-- well, viruses or instances of
- antigens and bacteria-- and tag them for pick up by
- macrophages or other types of phagocytes.
- Those were the B cells.
- Then you have your T cells.
- And here I'm going to talk about T cells a little bit
- differently than I had in the last few videos.
- Just to give a little bit more of a nuance-- so there's two
- types of T cells.
- You'll say, hey, they are helper T cells and cytotoxic T
- cells-- and you're not wrong, but what I'm going to do is do
- a slightly different differentiation just so that
- you are familiar with these terms. So there's
- two types of T cells.
- All T cells have T cell receptors.
- But they also have these other proteins on them and some of
- them have these proteins called-- CD4 proteins and some
- of them have what's called CD8.
- So this one right here would be called a
- CD8 positive T cell.
- It has the CD8 proteins on it.
- And this would be called a CD4 positive T cell.
- I've never used these words before.
- You're like, gee, where are these coming from?
- Now, the CD4 receptor is the thing that wants to bind--
- this is the thing that helps to go to the MHC II complexes.
- So most CD4 T cells are helper T cells.
- Most of the time-- and I want to make it very clear--
- immunology is a very-- I mean, this field, people are
- discovering things on a regular basis.
- So people are still understanding these things and
- there's all sorts of special cases, but usually when people
- talk about CD4 positive T cells, they're talking about
- helper T cells.
- So this is normally going to be a helper T cell-- or you
- could call it T helper, just like that.
- Likewise, the CD8 proteins, these are attracted
- to the MHC I complex.
- This is what brings them to the cells that have the
- cancer, that have expressed antigens
- on their MHC I complex.
- So most of the time CD8
- positive T cells are cytotoxic.
- And oftentimes, before a cell gets activated, they just
- describe it as a CD4 T cell or a CD8 cell and after it
- becomes activated and starts wanting to kill things, then
- maybe you call it cytotoxic.
- But this is all wordplay.
- I think you get the general idea.
- But just to remember what they do, this guy-- we just said he
- wants to bond to the MHC complex, so you have MHC
- complex plus presenting some antigen.
- This is MHC I right here.
- We learned in the last video, every nucleated cell in the
- body expresses an MHC I complex.
- So this is the case where something wacky is happening
- inside this cell.
- Maybe a virus has infected it.
- Maybe it's cancerous.
- It needs to die, otherwise it's going to keep producing
- viruses if it's infected by a virus, or otherwise it's going
- to keep dividing if it's a cancer and infect
- the rest of the body.
- So the CD8 kills infected cells.
- I'll just say bad cells because I don't if you can--
- cancer really isn't an infection.
- Kills bad cells-- cells that are-- if you don't kill them,
- they're going to keep producing viruses or keep
- splitting and spreading the cancer, while T cells--
- they're attracted to professional antigen
- presenting cells.
- And I always do a dendritic cell right here because those
- are the best antigen presenting cells.
- And they have MHC II complexes and it's digested some antigen
- and it presents it right there and then that activates the
- helper T cell.
- And then when the helper T cell-- all of these guys, once
- they're activated, they all go into effector-- they all start
- differentiating into effector and memory cells.
- An effector helper T cell does a couple of things.
- So if we're talking about a helper T cell, it can activate
- B cells and it also releases cytokines.
- Let's say this guy gets activated.
- He'll also start releasing these chemicals, which are
- really those alarm bells that tell other people to really
- get in gear-- maybe B cells and cytotoxic T cells start
- proliferating more rigorously.
- Actually, part of the cytotoxic T cell activation
- can be assisted or kind of given a boost by these
- cytokines-- of so by these alarm bells.
- So this guy's the alarm ringer, while the CD8 cells or
- the cytotoxic T cells-- in their effector
- mode, they kill cells.
- And of course in the memory mode, there's just a bunch of
- copies of these originals around that are ready and more
- than they were originally were, so that in the future if
- something like this or something like this occurs,
- they're going to be activated faster because they're going
- to be bumped into faster.
- So hopefully that clears up a little bit and I introduced a
- little bit more tough terminology, but I really want
- to stress my wife's point because she said, hey, you
- don't want people out there saying B cells produce
- antibodies even though it is effector B cells-- activated B
- cells that have differentiated into effector B cells.
- Those are what are producing antibodies-- because when they
- go to medical school, people are going to want to hear
- plasma cell.