Professional antigen presenting cells (APC) and MHC II complexes | NCLEX-RN | Khan Academy — Transcript

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1. 0:00In the very first immunology video, where we talked about
2. 0:02nonspecific defense mechanisms, we said if we had
3. 0:05some type of a pathogen-- let's say it's a bacteria--
4. 0:09that our phagocytes can recognize either proteins on
5. 0:14the bacteria or maybe it was some other type of pathogen--
6. 0:16some other marker on the pathogen.
7. 0:18It doesn't know what type of pathogen it is, but that's
8. 0:20enough for the phagocyte.
9. 0:23It'll engulf the pathogen.
10. 0:25So let's say this is the phagocyte.
11. 0:28Somehow one of their receptors touches one of the proteins
12. 0:32that's on the surface of this bacteria in this case and
13. 0:36says, gee, that's not a good thing to have around and so
14. 0:38it'll engulf it.
15. 0:39So the membrane is just going to surround the pathogen-- and
16. 0:44this part is going to go in.
17. 0:46It's going to kind of pinch in and the pathogen is going to
18. 0:48get surrounded and get engulfed--
19. 0:50phagocytozed by the phagocyte.
20. 0:53So the end product-- we saw this on the first video where
21. 0:56I talked about phagocytes-- is we're going to end up-- so
22. 1:00then the membrane's going to be completely around.
23. 1:02This is going to pinch in and bubble around this thing and
24. 1:05so we're going to have the pathogen sitting
25. 1:08inside of the phagocyte.
26. 1:11It's going to be surrounded by a membrane.
27. 1:13That membrane's called a phagosome.
28. 1:16And we learned there's different types of
29. 1:17phagocytes-- could be a macrophage, could be a
30. 1:22neutrophil, could be a dendritic cell-- and we have
31. 1:28it like this and then we also saw in that video, it's not
32. 1:30just done-- the macrophage doesn't
33. 1:32just digest this thing.
34. 1:34That by itself is very useful.
35. 1:35It got this bacteria out of the way.
36. 1:38If this was a virus, it got that virus out of the way.
37. 1:40But it does even more than that.
38. 1:42It takes that and then it lyses it-- or it breaks it up.
39. 1:45It doesn't have to actually use-- well, it breaks it up.
40. 1:50There's several ways it can do it, but the bottom line is, it
41. 1:53processes it.
42. 1:58It'll break up the pathogen-- and we saw that first video on
43. 2:01phagocytes that we'll actually have a lysosome bond to it and
44. 2:05dump all sorts of particles that are going to break up and
45. 2:08cut up this bacteria, in this case, into
46. 2:10its constituent molecules.
47. 2:18And then some subset of the leftovers, particular chains
48. 2:24of the peptides-- and remember, proteins are long
49. 2:27chains of amino acids.
50. 2:28Polypeptides are short chains.
51. 2:30So you take short chains of those and they're going to
52. 2:32bond to special proteins.
53. 2:37And this is essentially the topic of this video.
54. 2:39So let's say they bond to that special protein, bond to that
55. 2:42special protein.
56. 2:43And then those proteins get transported or get to the
57. 2:47membrane or the outer surface of the cell and they present
58. 2:49themselves along with the piece of the pathogen.
59. 2:53So the end product after phagocytozed this pathogen is
60. 2:57that the phagocyte will look something like this and it
61. 3:01will have these antigen presenting proteins, I guess
62. 3:08we can call them, that had bound to parts of that
63. 3:11original pathogen.
64. 3:13Let me do it right here.
65. 3:16So it has a little bit of the original pathogen on it that I
66. 3:20drew in green right here.
67. 3:23And these proteins right here, these are called-- it's a
68. 3:26fancy word and I talked about it in the previous video--
69. 3:29these are called major histocompatibility complexes,
70. 3:47or MHC for short.
71. 3:53And when we're talking about phagocytes or macrophages or
72. 3:58dendritic cells that are particular cases of
73. 4:00phagocytes, the major histocompatibility complexes
74. 4:03that they present after they've digested this
75. 4:05molecule-- this is an MHC class II.
76. 4:09Let me write this down.
77. 4:11This is an MHC class II protein.
78. 4:16It might seem like I'm really going into the minutiae of
79. 4:18what these proteins are, but we're going to see this is key
80. 4:21for activating other parts of the immune system, especially
81. 4:25the cell mediated parts of the immune system.
82. 4:27So this was the case with a macrophage or a dendrite.
83. 4:30They engulf something, they chew it all up, and then parts
84. 4:34of the chewed up thing that they ate, they attach to these
85. 4:36MHC II proteins and these MHC proteins go to the
86. 4:41surface of the cell.
87. 4:42The same thing-- or actually not the quite the same thing--
88. 4:45a very similar thing happens with B cells.
89. 4:49So if we have a B cell-- that's a good color.
90. 4:52B for blue.
91. 4:53We know B stands for bursa, but it could stand for bone
92. 4:57marrow just as well.
93. 4:58Let's say we have a B cell and it's got its membrane bound
94. 5:02antibody on it.
95. 5:03Remember, it's very specific to that B cells.
96. 5:07So all of the membrane bound antibodies, all 10,000 or so
97. 5:10of them on this B cell, they all expressed the same
98. 5:14variable part.
99. 5:16So this is a particular B cell.
100. 5:18So remember, this was nonspecific.
101. 5:20When we talked about phagocytosis, these guys just
102. 5:23say, you're a bacteria.
103. 5:24You're a virus.
104. 5:24I don't know what kind you are.
105. 5:25I'm just going to eat you up.
106. 5:26You look shady.
107. 5:27I'm going to eat you up.
108. 5:28I don't know what type you are or whether
109. 5:29I've seen you before.
110. 5:30When we're talking about B cells, we're talking about the
111. 5:32adaptive or the specific immune system.
112. 5:35And so these-- the variable ends of these membrane bound
113. 5:39antibodies are specific to certain parts of certain
114. 5:43pathogens, to certain epitopes.
115. 5:45Remember, epitopes were the parts of certain pathogens
116. 5:49that these specific chains can recognize and bond to.
117. 5:53So let's say that we're dealing with a
118. 5:55virus in this situation.
119. 5:57And let's say the virus just happens to
120. 5:59bond to this B cell.
121. 6:01Remember, there might be other-- in fact, there
122. 6:02definitely are tons of other B cells around, but their
123. 6:08variable portions are all different.
124. 6:17And that's what I always find amazing about B cells is that
125. 6:19they come from the same genetic line, their genes have
126. 6:23been shuffled around in their development so that they can
127. 6:26produce billions of combinations of these
128. 6:29proteins, or the variable ends of their antibodies.
129. 6:34So let's say we have some virus.
130. 6:37Let me say it's a bacteria.
131. 6:38When I did the other B cell example, I said we're dealing
132. 6:40with viruses.
133. 6:41But let's say some new bacteria and just some part of
134. 6:46its surface just happens to bind to only this B cell--
135. 6:50because this B cell has just the right combination.
136. 6:52So some part of his surface binds just to that B cell
137. 6:57right there.
138. 6:57That part of the surface that binds, remember, that was
139. 6:59called the epitope.
140. 7:01That's that part of the pathogen that binds to our
141. 7:05variable sequencer.
142. 7:06It won't bind to this B cell or this B cell because they
143. 7:09have different sequences here, but it binds to this B cell
144. 7:12and then that starts the activation process.
145. 7:15We'll go into-- sometimes this by itself can get the B cell
146. 7:19activated, but you normally need help from helper T
147. 7:22cells-- and we'll talk more about that.
148. 7:24And we said, once this happens, once you get
149. 7:27activated or the activation process starts, this guy
150. 7:30actually gets engulfed-- and I didn't talk about that in the
151. 7:32last video just because I didn't want to go into too
152. 7:34much detail.
153. 7:35So this whole thing gets engulfed by the B cell.
154. 7:42And then when it gets activated, it proliferates
155. 7:46itself and you normally need the T cells there and some
156. 7:49part of them become plasma B cells, some part of them
157. 7:52become memory B cells.
158. 7:58Remember, the plasma B cells say, gee, I've been activated.
159. 8:02I'm just going to produce a ton of these antibodies.
160. 8:05So the plasma B cell will just produce tons of these
161. 8:07antibodies and start spitting them out so that they can
162. 8:10attach to more and more of that pathogen and just mess
163. 8:14them up in different ways, either tag them so that other
164. 8:18macrophages or phagocytes eat them up-- or tag two of them
165. 8:23so you bundle them up so that they can't operate properly.
166. 8:25Whatever.
167. 8:25I'm not going to go into detail on that.
168. 8:27That's when it gets activated.
169. 8:28But the interesting thing is that the B cell will also do
170. 8:31what the phagocytes do.
171. 8:33The B cell will also take this guy into the cell.
172. 8:41Maybe he's initially attached to the antibody-- and break
173. 8:47him up, take pieces of this pathogen and attach it to MHC
174. 8:55II proteins and then present them on the surface.
175. 8:59So a B cell will also present the antigen.
176. 9:06So this is also an MHC II complex-- a major
177. 9:11histocompatibility complex-- and just so you know, histo
178. 9:13means tissue.
179. 9:14So this is related to whether something is compatible with
180. 9:17the tissue in your body and we'll talk more about that and
181. 9:19how it relates to transplants and all of that.
182. 9:21So this is also an MHC class II.
183. 9:25In both cases, whether we're talking about B cells that
184. 9:28recognize a very specific pathogen-- and it could be a
185. 9:31specific virus, a specific protein, a specific bacteria,
186. 9:37or in the cases of phagocytes, they'll just say,
187. 9:40oh, you look shady.
188. 9:41Let me take you in.
189. 9:42I don't know what type of bacteria or virus or protein
190. 9:44you are, but in either case, they both engulf them, take
191. 9:47pieces of them, cut them up, and present them on their
192. 9:50surface in a complex with the major
193. 9:52histocompatibility complexes.
194. 9:57So cells that do this are called professional antigen
195. 10:01presenting cells.
196. 10:05This is what they do for a living, although they do other
197. 10:07things, as we've seen.
198. 10:09The phagocytes eat things.
199. 10:10The B cells generate antibodies or memories so that
200. 10:14they can later be activated to generate antibodies, but these
201. 10:17are called professional antigen presenting cell.
202. 10:25And the antigen in question is this little piece of the
203. 10:29actual thing that you're trying to track, that little
204. 10:35piece-- the actual part of the pathogen, that's what the
205. 10:38antigen-- so it's presenting the antigen.
206. 10:40It's professional because it takes pathogens from in the
207. 10:48fluid of our system and then engulfs them, breaks them up,
208. 10:51and then presents them.
209. 10:53Now there are also nonprofessional antigen
210. 10:57presenting cells.
211. 10:58And in fact, most cells are this.
212. 11:00In fact, even these guys.
213. 11:02Actually, I'm going to wait for the next video.
214. 11:03I realize all of my videos are getting long.
215. 11:05So you're probably thinking, these guys in either case
216. 11:10engulf them, cut them up, present them,
217. 11:12what is it good for?
218. 11:13You'll see these MHC II, these are what are recognized by
219. 11:18helper T cells.
220. 11:20It'll all form part of the puzzle of how our immune
221. 11:23system works.
222. 11:24So in the next video, I'll talk about MHC I presenting
223. 11:28cells, which is pretty much all body cells.