Table of Contents
1. IntroductionScience class, more than any other class, requires you to learn how to act appropriately in many different situations and transition comfortably between them. In the course of a typical week, you will spend some of your class time doing all of the following:
Personally, I find that the wide variety of different types of activities makes science a lot of fun. However, it also means that we have to work hard at the start of the year to learn what type of behavior and noise level is appropriate for these different activities. In particular, I need to be able to be confident that you will be respectful of other classes when we are moving through the hallways to the science lab or the computer lab. If I start dreading labs because some students lose control when we're not in the classroom, I'll be a lot more reluctant to design lab activities, and class will become a lot less fun.
- Working independently on a worksheet or quiz
- Working through and discussing a particular problem in a group
- Participating in a whole-class discussion
- Listening and taking notes as I or one of your peers presents a lecture
- Working on an activity or experiment with your partner in the science lab
- Training with a computer program in the computer lab to build up fundamental skills
To train you in how science class works, I picked a topic that I personally find fascinating: the ways in which the cells in your blood cooperate to keep you healthy and fight off germs. Part of the fun of this is that you can see all these cells under a microscope, and you can recognize the parts of the cells that are their weapons or other adaptations for their defensive jobs.
2. Learning about blood cells
2.1. Using a microscope
If you have ever used a magnifying glass to look at something small, you know that it is rather tricky to get the thing "in focus." You have to get the distances between the object, the glass, and your eye exactly right; otherwise, what you see is all fuzzy and indistinct.
Learn to manipulate a microscope's focus, objectives, and slide.
- Getting in focus
- To the science lab!
- What did we see?
- Introducing tomorrow's project
This same "focus" issue happens with a camera: if the camera focuses on something close by, the background is all fuzzy, and vice versa. Your eyes work this way, although you might not notice it because your eyes are constantly adjusting so that whatever you are trying to look at is in focus. If you hold up a finger in front of your nose and focus your eyes on it, you will notice that the rest of the room becomes blurred.
In all these cases, we see that any tool for seeing things has a particular distance at which the image will be sharp, and any objects closer or further away will start to become blurred. This is a particularly big problem when looking through a microscope - which is what we'll be doing in a moment - because the things we will be looking at are so small that if they are even a tiny bit above or below the "focal point" of the microscope, they won't be visible at all. Therefore, a microscope has a very precise mechanism for adjusting the focus: a "focus knob" that gradually either moves the stage up or moves the microscope down so that you can carefully adjust it to focus perfectly on the slide.
When you're looking at something through a microscope, then, in order to see anything at all, you have to make sure that part of the slide with something on it is centered under the microscope, and then slowly move the knob while looking through the eyepiece. At some point, you will begin to see cells, and then you can adjust the focus knob to make the image as clear as possible. It might be a good idea to start by turning the knob as far as it will go in one direction or the other, so that you can be sure of covering the whole range.
So, just to review, this is how you get a microscope focused on a slide:
After we have all done this in the lab, we will learn how to do two other things: changing the "objective" to increase the magnification, and moving the slide around. I find that in order to move the slide a tiny distance in a controlled manner, it works best to press a finger against the stage next to the slide, then gradually roll your finger toward the slide.
- Make sure the light is turned on and the objective marked "40X" is selected.
- Center the slide under the microscope. The circle of light will help you.
- Adjust the focus knob all the way up or down.
- Look through the eyepiece. You should be able to see a circle of light.
- Slowly turn the knob. When you see cells appear (it will happen quite suddenly) you can adjust the knob to make them as sharp as possible.
For the sake of discussion, we will assume for now that all of the objects you see through the microscope are some kind of cells. You have probably heard that word before. Cells are the "building blocks" that make up all living things, and each cell is to some degree able to live in an independent fashion. The most common type of cell, by far, on these slides, are red blood cells, something you have probably also heard of before.
The dark part in the middle of some cells is called the nucleus. You don't need to know yet anything about what the nucleus is for, but it will be helpful to have a word to refer to it by.
Your assignment, at this point, is to answer a single question: How many different types of cells are there in blood? This is a hard question to answer, because you need to be able to decide if two cells that are very similar are actually different types, because of their minor differences, or the same. Other than that one question, the goal for today is just to get to the point of being able to see things through the microscope, under different objectives, and to be able to move the slide around.
Tomorrow, we will be investigating these same slides in more detail, trying to gather more details about the cell types you identified. At the end of class, we will brainstorm a list of different characteristics that it might be useful to note for each cell type. This is the part of class that is most important to take notes on, since this list will guide what you do tomorrow. Your list might include:
Our goal in the next class will be to come up with a definitive list of features that identify each type of cell.
- Cell color, shape, and texture
- Cell size (relative to red blood cells)
- Nucleus size, color, shape, and texture (if nucleus is present)
- How common that type of cell seems to be
2.2. Looking at blood cells
Yesterday you got a brief opportunity to see some blood cells under the microscope. We know very little about these cells so far; we have just gotten as far as saying that all the objects you were looking at were, in fact, cells of some sort, that ones you saw the most of are called red blood cells, and that the dark part in the middle of some cells is called the nucleus. You don't really need to understand much about these different terms yet; we just need some basic vocabulary just to make it possible to talk about what we are seeing.
Classify blood cells according to their appearance under the microscope.
Your assignment for today is to work with your lab partner to decide what groups to split the cells up into, and collect as much information as you can about each type of cell. You can use this Cell Notes worksheet to record your information. You will probably need two sheets to list all the cell types.
For most of the cells you will not yet know what their name is. The "sketch" part, obviously, contains almost all of the information that you will record elsewhere in words, but if you're like me and not very good at drawing, the words might be a better description.
When we get back from lab, we will briefly discuss our results, including how many different types of cells each group found.
2.3. Germs: the Bad Guys
The stick-like things you saw in the image in the do-now are a particular type of bacteria. Bacteria are tiny living things made up of just one cell; so, you are looking at dozens of bacteria just in that one picture. A single bacteria is called a bacterium.
Understand what different kinds of germs the body needs to fight.
- Go over do-now
- The mystery: "Cursed" fields
- Koch and his microscope
- Other types of germs
- What do you think the white blood cells in the picture are doing?
- What are the bacteria doing to protect themselves?
Because bacteria are so tiny - smaller even than our own blood cells - it took a long time for people to begin to be convinced that they could actually do us any harm. The picture here shows the first bacteria that was ever proven to cause a disease. Before I tell you that story, I'll give you a little bit of background.
In Europe in the 1800s, farmers were convinced that some fields were cursed. There were always a few fields in each village in which, if you brought your cows there to graze, you could expect that some of them would suddenly start acting listless one day, and be found dead the next morning. The blood of animals that died this way was almost black, rather than a healthy red, and their meat was poisonous.
Robert Koch was a doctor in Germany who had just gotten a present from his wife: a microscope. In those days, it was rare for a doctor to even have seen a microscope; they were not commonly used in medicine at all. Koch became interested in the question of what was causing this cow disease, which farmers called anthrax. He looked at some of the blood of cows who had died of the disease, and he found the strange rod-shaped things you saw in the do-now.Now, many people had seen these same stick-like cells in anthrax blood before, but most doctors agreed that that was just a sign of the disease, not the cause. Now, clearly those sticks had to be generated inside the body, rather than coming in from somewhere else. There was no way that billions of cells, tiny though they may be, could suddenly appear inside a cow for no reason. But when they were observed under a microscope, they didn't seem to do anything; they just sat there, inert. So how could they be dangerous? Perhaps, the doctors said, the red blood cells in cows that were dying of anthrax for some reason collapsed into stick shape, or else some organ was breaking down and letting those strange cells out into the blood.
Koch wasn't so sure that those sticks were harmless. To see if they were dangerous, he tried injecting a little bit of cow blood, full of those sticks, into a mouse. The next morning, when he came in to his office, the mouse was dead. Koch was sure, then, that he had proved that there was something dangerous in the cow's blood. But the real surprise came when he dissected the mouse and looked at its blood under a microscope: everywhere he looked, he found thousands of those same black sticks. Clearly there were many, many more now than that tiny bit he had injected into the mouse. But were the sticks multiplying on their own, or were they getting cells in the mouse to turn into copies of themselves?
To figure that out, Koch decided that he had to be able to see what was going on inside the mouse, while the disease was growing. But how to see inside a living creature? It was not enough just to put some of those sticks into healthy blood and see if they would change at all; one could wait days and still observe no change. Finally, it occurred to Koch that perhaps it was necessary for the blood to be heated to the body temperature of an animal in order for the sticks to multiply. He created a very clever slide in which a drop of healthy cow blood, with a little bit of tissue from a mouse dead of anthrax, was stuck in a well between two glass slides. Then, he sat down to watch what would happen.
Koch's original plan was to make observations of the same spot every hour, noting the number of sticks visible and sketching what he saw to compare to the next hour. He locked himself into his lab with the intention of spending the whole night making minute observations to try to catch the progress of the disease. In the first few hours, he spent fifty minutes out of the hour staring though his microscope, but didn't see any change. He went off for a brief walk to rest his strained eyes. When he came back and looked through the microscope, he jumped back in surprise and fear - something was moving under the microscope! He could actually see the individual segments of the sticks stretching themselves out, then suddenly splitting in two. Before his fascinated eyes, the few loose sticks floating around in the blood grew, in only a few hours, into a dense mess of strands that looked like a tangle of yarn.
Thus Koch became the first person ever to prove that germs could cause disease. The anthrax bacteria, he realized, were not harboring any hatred toward cows, but it was in their nature to grow, when they had the right temperature and the right nutrients. And unfortunately, the cow's blood provided exactly the right temperature and nutrients for it to grow like crazy, until those tangled skeins of bacteria choked out the cow's arteries.
Bacteria are all around us: they cover our skin, they are found all around in our mouths and our digestive tract; there are actually ten times as many bacteria cells in your body as there are human cells. But most of the time, they are kept in control by your white blood cells, which attack them whenever they try to go somewhere in your body where they don't belong.
2.4. Cell Types
We will start off class by trying to answer the question, "What do we know about what blood does and what is in it?" Depending on the class, we may come up with a list something like this:
Understand the different jobs that are done by red blood cells, white blood cells, and platelets.
- Red blood cells
- White blood cells
- Cell membranes
You already know that the most common cells you saw on our blood slides are red blood cells. Red blood cells are very simple cells. As you saw, they don't even have a nucleus, which is essentially the "brain" of the cell. They are really just mindless sacks of hemoglobin that float along in the blood, not able to take any action of their own. But there are a lot of them, because they do the primary job of blood: getting oxygen from your lungs to the rest of your body and then bringing back carbon dioxide for you to breathe out.
- Blood carries oxygen from the lungs to the muscles and carbon dioxide back.
- It is red blood cells that do this job.
- When you cut or scrape yourself, first blood comes out, then just pus, then a scab forms.
- Blood can have germs in it (it is unsafe to touch a stranger's blood)
- White blood cells fight against germs.
If you happened to notice some really tiny cells on your slide, you were looking at platelets. Most of the time, platelets don't do much; they just circulate around in the blood. However, when you get a cut, the skin near the cut releases a chemical that causes platelets to suddenly become very sticky, grabbing and holding on to cells around them to form a solid mass known as a clot. This is what scabs are formed out of. If our blood didn't clot like this, we would be in danger of bleeding to death even from very minor injuries. (There is actually an inherited disease called hemophilia that does exactly this)
The rest of the cells you saw are, collectively, known as "white blood cells". You might be confused about why they would be called this, when many of them are very deeply colored in pink and blue. The reason is that the colors you saw on these slides are artificial, the result of staining chemicals added to the slide to make white blood cells visible. Here's what blood actually looks like when it isn't stained:
Can you spot the white blood cells?It was discovered fairly early on that red blood cells carry oxygen, because their color changes slightly when they are carrying oxygen versus when they are carrying carbon dioxide. It took a lot longer to explain what job, exactly, was done by the white blood cells. Elie Metchnikoff, a Russian scientist, was the first to advance the theory that white blood cells are there to fight against disease. Looking under a microscope at starfish eggs, which, conveniently, are transparent, he noticed that if he stuck a bit of rose thorn into the egg, white blood cells would gather around it as if trying to repel an invader. Later, he observed white blood cells actually consuming entire bacteria.
Metchnikoff theorized that white blood cells are the policemen of the circulatory system. Somehow, he said, they were able to recognize when nearby tissue was inflamed as a result of the presence of some invading substance or germs, and they can move at will through the body to seek out these threats. The yellowish pus that you might see coming out of a wound, particularly a large but not very deep wound like a skinned elbow or knee, is almost purely composed of white blood cells, forming a solid barrier against bacterial invasion. You can think of this like the lines of police with shields and helmets who stand at the edge of a rioting crowd to keep them from attacking some person or building.
If you're a germ, this is what you have to get through in order to enter the body through a pus-covered wound.Ironically, even though Metchnikoff was exactly right about what white blood cells do, his research was ignored for years because he was Russian, and other countries' scientists only wanted to support ideas originated that their own countrymen had come up with. The main competing idea at the time was that blood serum, the liquid part of blood, was somehow inherently deadly to bacteria. We now know that both ideas were correct; the body uses a wide range of overlapping defense systems to do its best to ensure that no bacteria get through.
Metchnikoff also observed, as we mentioned before, that one of the primary weapons white blood cells have against bacteria is their ability to "eat" a bacteria. It is kind of difficult to understand how this is even possible, since a white blood cell has no mouth. The key idea to understand is that the skin of the cell, the cell membrane, is actually all mouth, since it is able to stretch itself into any shape, allowing it to wrap around a bacteria and then even to "pinch off" the wrapped-up bit so that the bacteria is left wrapped up in its own little wall of cell membrane, isolated inside the cell:Incidentally, this same sort of process also makes it possible for cells to split in two, simply by narrowing themselves off at one point and then pinching off the connection:Listening to me lecture and taking notes for nearly a whole period isn't a lot of fun, I know. But there is one redeeming aspect to it: I will always make sure to end the lecture part early in order to let you ask any questions you have. While I'm talking, if you come up with any really good questions - questions that would fill in a gap in what I said, or take it one step further - you should make sure to scribble those questions down somewhere to ask at the end. If you can ask a good question, you'll get a lot of respect from me, because that is the surest sign that you were really paying attention. The other fun part about asking questions is that the answer won't be something you "really" need to know - just something I'm telling you because it's interesting to know - so you don't have to worry about taking notes or memorizing it for the test.
It might not be clear exactly what counts as a "good" question, so for today I've prepared a "good" question to ask myself in case no one comes up with one of their own. My questions is, "Mr. Z., where do new blood cells come from?" This is a good question because it not only shows that you were paying attention, it shows that you were thinking too. I told you that these cells are leaking out all the time when you are injured. You also know that people build up a bigger volume of blood as they grow older, and that endurance athletes like Lance Armstrong grow lots of extra red blood cells. You could probably even guess that blood cells wear out, for the same reason that dollar bills in "circulation" in the US have to be periodically replaced. So where are the new blood cells made, and how do they get into the blood?
Blood cells are actually made, surprisingly enough, inside of bone marrow (that is, unless you're a chicken. Birds have a special organ called the bursa, down in their tail, that does the job of producing blood. If you think for a moment, you can probably figure out why birds don't do this in bone marrow). Bone marrow is the spongy, meaty part inside your bones, particularly big long ones like your thigh bones and upper arm bones. Actually, blood cells don't last very long at all. Red blood cells last about four months, and start looking a bit tattered by the end from all that ceaseless moving about, like a one-dollar bill that's been in circulation for ten years. White blood cells much last shorter times, a few weeks at most, since they are involved in active combat with invaders. Your spleen is constantly filtering out cells that are getting too old, and at the same time new blood cells are constantly being made in your bone marrow.
2.5. Defensive Cells
When the army of one country tries to invade another country, the defenders are faced with a difficult dilemma. The most effective response, it might seem, would be to put as much of your army as you can right up on the border, so as to prevent the invaders, at all costs, from entering into your country. But the weakness of this strategy is that if the enemy manage to breach that defensive line at even a single point, they will then be able to bypass the rest of your defenses and have free run of the undefended territory beyond.
Understand that your body uses a wide range of overlapping strategies to kill invading germs.
- Defense in Depth
In modern warfare, the concept of defense in depth is seen as a key guiding principle. The idea is to have several lines of defenses, with each line receiving support from the one behind it, but with each line also able to mount a defense on its own. So, for example, in a castle, the inner keep is not worthless in the initial attack, because it is tall enough that defenders can fire arrows over the walls of the outer defenses. However, if the invaders do somehow manage to get inside the outer ring, they are then faced with a second, even more formidable set of defenses to overcome.
You body's defenses against germs work in the same way. Your skin keeps out 99% of germs, but it would be foolish to rely on that alone, so there are also systems for fighting off germs once they get inside the body. And those systems, likewise, are layered: instead of relying on one single strategy, you body has dozens of different ways to resist infection, with each of those ways able to operate in isolation if necessary or in cooperation with the others if possible.
In the last class we saw what is probably the most straight-forward way your body resists germs: nearly all types of white blood cells have the power to engulf any dangerous bacteria that they encounter. But how do cells know what is a dangerous bacteria and what is not? What do they do to kill the bacteria once they've eaten them? How do they warn other cells about what is going on? How do they deal with an invader that is much, much bigger than them?
We can start out just by brainstorming some ways that one cell might try to attack another. Here we can work off of our understanding of cells as flexible sacks of liquid swimming through blood. Our list of ways a white blood cell might attack a bacteria could look like this:
The most straightforward way to answer these questions is to go one by one through the types of cells we have seen, learn their names, and learn what strategies they use to attack bacteria. It turns out that a lot of the features of cells that we saw through the microscope will turn out to be relevant to their role in the body's defense system.
- Eat it. (We already knew this one)
- Shoot it with a torpedo (like a submarine!)
- Blast it with depth charges (again, using the submarine analogy)
- Pop it like a balloon (or otherwise try to cut it open)
- Poison it
Neutrophils - the body's riot police
The most common type of white blood cell that you saw on your slides is called a neutrophil. These are the cells that make up most of pus, because they are the "first responders" to accidents in the body, moving quickly to the site of a cut or scrape just like a police car or ambulance putting on its siren to cut through traffic. Instead of a "radio" system to let them do this, there is a chemical signaling system that the body uses: inflamed skin, skin that gets red and tender, is giving off a "smell" that attracts neutrophils. It was neutrophils that Metchnikoff saw moving toward the thorn he stuck into a starfish egg.
The primary job of neutrophils is to not let anything in. Remember the picture of riot police from yesterday, explaining what pus is from the point of view of a bacteria? It is neutrophils that make up most of pus, filling in any opening in the skin with literally millions of "riot police" all on the lookout for anything suspicious.
How do neutrophils fight against bacteria? Like all white blood cells, they can simply engulf the bacteria. But this leaves the bacteria still alive, merely trapped inside a vesicle in the cell ("vesicle" is the name for any part inside the cell that is separated off by being enclosed in its own membrane). The bacteria is nicely isolated from the cell; it can't break out through that membrane and cause any damage. But there are no prisoners taken in this battle of cells; it is fought to the death. How does a neutrophil kill off a bacteria, once it has it trapped inside itself?
You may have noticed when you were looking at neutrophils under the microscope, or looking at these pictures now, that they are not a single, smooth color: there are little colored speckles all through the cell. This granular appearance results from lots of little vesicles containing high concentrations of various chemicals. Just as the vesicles containing bacteria are there to keep the bacteria isolated from the rest of the cell, the purpose of these vesicles is to keep those chemicals, which are very dangerous ones, out of the rest of the cell. You might think that whatever chemicals a cell would use to fight with would be something you had never heard of before, but actually some of them are quite familiar:Once the bacteria is inside the neutrophil, it can be killed off simply by merging its vesicle with a vesicle containing one of these poisons. The dead bacteria and the remaining poison are then expelled from the cell by merging that vesicle back into the main membrane of the cell. It is also possible for a neutrophil to release this poison into the blood around it in order to kill off bacteria without needing to engulf them first, but this approach is somewhat troublesome for two reasons: because the poisons hurt the nearby body cells (it is this that makes inflamed skin painful), and because the bacteria, if there is nothing holding them in place, can simply swim away from the poison.
Eosinophils - the body's heavy artillery
The next type of white blood cell that you saw in blood is called an eosinophil. You might notice that they look a lot like neutrophils, except that their granules of poison are much bigger. If that led you to guess that they go after something larger than your ordinary bacteria, then you were right - eosinophils are the heavy artillery of the body, or perhaps the big game hunters. Bacteria, which neutrophils attack, are many times smaller than them; eosinophils, on the other hand, tend to attack large, big enemies, like parasitic worms. Because they can't engulf such an adversary, they have to rely on releasing poison near it, which tends to be very damaging to nearby body cells.
If you have ever had a severe allergic reaction in your skin, such as a poison ivy rash, then you were seeing what happened when eosinophils saw the poison ivy oil and decided that it smelled like some sort of parasite. The damage from the rash is actually caused by your body's own defense systems! Mounting this sort of extreme response when there isn't actually any danger there is the price your body pays for being able to respond effectively to an actual parasite, some type of which "smell" very similar to poison ivy.
Monocytes - the body's detectives
This type of white blood cell, called a monocyte, would on the surface of it seem not particularly useful. Like all white blood cells, they can engulf invaders, but you will notice that they don't have any large number of granules of poison to dispatch those invaders with. What they do, instead, is to hold on to the "bodies" of the bacteria they have killed, chop those bodies up into pieces, and show those pieces to lymphocytes (the last kind of white blood cell) who decide if that type of bacteria is something particularly nasty that requires an aggressive response from the whole body. You might have noticed several empty vesicles in the pictures above or in the monocytes on your own slides; these contain dead bacteria, which, instead of being expelled back into the blood, are being carried around so that the monocyte can consult with a lymphocyte to determine how dangerous they are.
Monocytes also have the job of cleaning up the dead bacteria that are left over after the neutrophils have been through. If the dead germs were just left where they dies, they would provoke constant panic among the cells nearby, who can't easily smell the difference between a live germ and a dead one. This scavenging of dead bacteria also allows the monocyte to bring them in to the lymph nodes to consult with the lymphocytes.
You can think of monocytes as being like police detectives: they collect clues and bring them back to the police station to be analyzed, in order to try to figure out if a big crime is underway. The "police stations" in this case are the lymph nodes. When you are sick and the doctor feels under your jaw, they are trying to feel two lymph nodes there; if they are swelled up, that it proof that they are in alert mode, and thus that your body is actively responding to an infection that it recognizes as dangerous.
Lymphocytes - the body's memory of its enemies
As you might guess, this last type of white blood cells are the lymphocytes. Their job is to recognize and respond to threats that the body has faced before. We have already mentioned that when a monocyte has killed a bacteria, it takes the body back to the lymphocytes, in the lymph nodes, to have them check if it is something dangerous, something that might be causing an infection. But why, then, are there lymphocytes in the blood?
If a lymphocyte decide that there is an infection underway, it respond to this by duplicating itself into hundreds of copies, and then dispersing into the blood stream with a single mission: to locate the invader and mark it for destruction. Rarely does a lymphocyte do the dirty work of actually eating an invader; as you can tell from the picture, they really don't have the space free to eat anything. But what they do instead is to mark their target bacteria with chemicals that will single them out to be attacked by other white blood cells, particularly neutrophils. You can think of this like taping a "kick me" sign on someone (although in this case, the sign says "Engulf me and poison me to death!") Another analogy would be to compare it to police putting out an all-points bulletin on someone, or planting a GPS tracking device on their car.
If you watch a lot of cop shows, you might like the analogy that lymphocytes are like the feds. If there is a particular crime or criminal involved in a case that they have an interest in, they swoop in in large numbers and take over the investigation. But otherwise, they don't do much in terms of everyday policing. It isn't their jurisdiction.
I had a bit more trouble thinking of a sample "good question" for this lecture. My first thought was, "Mr. Z., do white blood cells ever team up to take on some particularly nasty enemy?" But then it occurred to me that really what I've been telling you this whole time is that, yes, that is exactly what they do. Neutrophils gather around a scrape or cut (or a thorn in a starfish egg, in Metchnikoff's experiment) in massive numbers in order to overwhelm any bacteria that might try sneaking in. Eosinophils gang up on large parasites in the same way, which I didn't say but which would probably be easy to guess. And the whole system of monocytes, lymphocytes, and neutrophils works together to identify germs that might cause infection and mount a coordinated attack on them all through the body.
A "smudge cell"
Another question I thought of was, "Mr. Z., do cells ever physically attack each other?" You might imagine that, if you could poke a hole in another cell, it would pop like a balloon. This is actually true, but since cells are rather squishy and rounded, and move rather slowly, they can't just extrude a sharp arm and poke another cell with it. Instead, when a white blood cell needs to kill another cell that is as big as it (such as a body cell that has cancer), it sidles up close to that cell, and then releases into the space between them the contents of a special type of poison vesicle that contains - well, let's call them little soldiers for now - that go over to the membrane of the other cell and open up a hole in it. Then the neutrophil detaches, and water goes rushing in to the other cell until it pops like a balloon. (You might occasionally see on a blood slide a cell that has been a victim of this kind of attack; it sometimes has an intact nucleus but otherwise is all smeared around, with no clear border. Cells like this are called, appropriately enough, "smudge cells")
Another good question would be, "Mr. Z., how do blood cells die?" This is a good question because I definitely implied in lecture that cells can die, but I didn't explain what that would mean. Unfortunately, it would be hard to explain at this point what causes cells to die, because we haven't yet gotten to the point where we understand what makes a cell alive. As a rough approximation, we could say that a cell is like a city, with a bunch of workers all trying to keep the city going. If enough of the workers are killed off in one of the key industries - say, power generation, or manufacturing - then the cell economy collapses and it stops functioning or lapses into anarchy. Body cells can also go into a process called "programmed cell death", where another cell politely asks you to die and you do so, in a way far less messy than exploding or collapsing into internal anarchy. This is the fate that awaits most blood cells and well behaved (non-cancerous) body cells.
We've covered a lot of material today. Here's the outline I put up on the board and which I hope is now in your notes:
- Neutrophils (the riot police)
-Job: keep bacteria out
-Move quickly toward any break in skin; pus is made of them
-Can engulf bacteria
-Carry vesicles filled with poison to kill off bacteria
- Eosinophils (the heavy artillery)
-Job: attack parasites
-Carry much larger poison vesicles, which they release into the blood near the parasite
-Allergic reaction happens when they attack something harmless and damage body cells instead
- Monocytes (the detectives)
-Engulf invaders or scavenge dead germs, then bring their bodies back to show to lymphocytes
- Lymphocytes (the memory)
-Recognize particular invaders and start a body-wide response to combat infection
-Each can dispatch hundreds of copies of itself into the blood stream to seek out a particular invader
-Rather than attacking invader directly, it marks it to be targeted by neutrophils
3. Applying what we learnedIn science class, you are expected to think like scientists. You should be able to take the information that you now have about blood cells and use it to reason out what is going on in all kinds of different situations.
The analogy that I gave you for this is that of building with blocks. In order for you to build something, first I have to give you some blocks; then the fun begins as you try out different ways of putting them together. In this class, the blocks are facts: I will give you the facts you need to understand some subject, but then we will start the fun part, which is putting those facts together in new and interesting ways, using those facts to explain what is going on in some unfamiliar situation.
At this point, I have given you all the facts that you need about blood cells. You know about six different types of cells and what job each one does. Now that we know that, we're going to look at a bunch of situations in which these cells are acting, and we're going to try to use our understanding of the fundamentals of blood cell function to understand, explain, and make predictions in all these situations.
3.1. Lymphocyte Training
We know already that lymphocytes have the job of recognizing dangerous, infectious germs. Today we will discover a bit about how they are trained to do this, and about what other roles they play in the body.
Practice reasoning based on what we know about blood cells.
You probably know that it is not necessary to get a disease in order to train your body to fight it; you can also get a vaccine for that disease. Today we'll use what we know about blood cells to reason out why vaccines are effective and what their limits are.
Practice reasoning based on what we know about blood cells.
We know already that when your skin is broken, neutrophils come rushing in to defend you against invading germs, and platelets nearby start sticking together to form a scab. But how do the cells know to do this? How do they get out of your blood and into the skin? How does this connect to the classic symptoms of "heat, redness, swelling, and pain"?
Practice reasoning based on what we know about blood cells.
In the do-now, we will figure out how all the symptoms listed above are just manifestations of the same cause: your blood vessels are swelling up near the injury to help white blood cells access the injured site. Next, we'll see a video called The Inner Life of the Cell that is designed to explain, in minute detail, all the steps that are involved in a neutrophil recognizing inflammation and slipping off into the skin to deal with it. Finally, we'll apply these ideas to try to figure out what is going on in a situation where neutrophils are not reaching the site of an infection, and to discuss what the results of that would be for the body.
3.4. Lymphocyte Sequestration
At this point, you know enough about blood cells to be able to make sense of the way that scientists actually talk about them. So, we'll be listening to a podcast from Nature, one of the most well-known science journals, about a topic that ought to make a lot of sense to you. First, though, we'll be sure to discuss the extra vocabulary you might hear in this podcast, so that you will be ready to listen and understand.
Practice reasoning based on what we know about blood cells.
4. ReviewBecause I know you'll ask: Yes, the test counts toward your term average. Yes, I know that this is just a training period. I don't care - you still should be learning. My hope is that you are confident enough about the material we've learned - after all, it wasn't a very long unit - that you ought to be cheering at the idea of starting off the year with a grade from this unit already on your term average.
There isn't actually a day in class for review; we've been reviewing all week. However, just in case you are up on the website reviewing for the test, I have also outlined below what I expect you to know.
Remember that you are only responsible for knowing the things we learned in the first three weeks of the unit. The fourth week gave us a lot of opportunities to practice applying that knowledge, but you aren't required to remember, for example, how AIDS works or what LAD syndrome is. The thinking problems you will see on the test, like all the problems we did this week, can be solved with just the information above.
- Microscopes and Cells
- Parts of a microscope: objective, focus knobs, slide
- Recognize what a cell is and identify the nucleus in cells that have them.
- Understand the interesting stretchy nature of the cell membrane
- Understand how a cell can split into two, or engulf something to form a vesicle
- Types of blood cells (You should be able to identify these in color pictures)
- Red blood cells carry oxygen to the body and carbon dioxide back to the lungs
- Platelets are tiny cells that form scabs by gluing other cells together when signaled to do so.
- White blood cells all have jobs related to defending the body against germs
- Types of white blood cells (You should be able to identify these in color pictures)
- Neutrophils (the riot police)
- Job: keep germs out of the body
- Moves quickly toward any break in the skin; primary component of pus
- Strategy: engulf germ, then kill it by merging its prison vesicle with a poison vesicle
- Eosinophils (the heavy artillery)
- Job: attack large invaders such as parasites
- Strategy: release lots of poison near the invader
- Problem: poison also damages body tissue
- Monocytes (the detectives)
- Job: engulf invaders or scavenge dead germs and bring them to lymph nodes for analysis
- Often seen with empty vesicles containing digested germs
- Lymphocytes (the memory)
- Job: recognize particular invaders caught by monocytes
- Response: split into many copies of yourself that spread out in the blood to hunt down that invader
- Strategy: don't engage enemy; just mark them to be attacked by others
- Doctors know: inflamed lymph nodes means your body has recognized the threat and is fighting it
The notes are nicely summarized in the Blood Cells Notes.