CLASS: Immunology

Welcome to Immunology
As you have probably guessed, Immunology is the study of how our bodies defend themselves from harmful organisms and substances. In this unit, we will cover the myriad defenses of the immune system and the even more numerous threats they face. Our first section will be on phagocytes, a type of white blood cell. Second will be lymphocytes, another type of white blood cell. I have a blog post set up where you can ask questions about the first lesson, which you can read below. Even if you have read this page before, read all the sections again as a lot has been updated. Thanks!

Information:
I have posted some things on the downloads site that you may want to take a look at. There is a fairly comprehensive overview of the immune system as a whole. If you have recommendations or questions, please post your comments on my wikia blog page. Also, if you want to follow this course, please send me an email at [mailto:ozone96@gmail.com ozone96@gmail.com]. Also, my Skype name is thedefender96. Don’t ask. As for some actual immunology, I will give you the very basics of the immune system and then some information on phagocytes and phagocytosis. Basically, the immune system is centered on white blood cells. These are its soldiers, the ones who actually go around killing off gems left and right. These fall into two major categories: Phagocytes (which we will go over in a minute) and Lymphocytes (Which are in the lesson after). Each of these categories in turn includes dozens of different types of cells, especially the lymphocytes, that all have different functions. The body also uses chemicals to hinder pathogens and alert the immune system when there is an infection. We will also discuss these later on. The lymphatic system is where most of the body’s white blood cells hang out (hence lymphocyte). It is part of the circulatory system, but uses separate vessels and also has lymph nodes, where lots of cells and chemicals are produced. If at any point in this class you are confused by fancy terms, look at the Vocabulary section.

Phagocytes
Now, about phagocytes. Phagocytes are generally the first into the fray in an infection. Their first task: eat. The way they go about this is they randomly move about. Antigens are critical to all immune system processes. First and foremost, immune cells can detect the antigens of cells, so when the antigen is that of one of your own cells, they know not to attack. However, should they encounter a foreign antigen, red alert! As you will see, antigens are crucial to every part of a phagocyte’s task. First, it detects a foreign antigen. It then uses pseudopodia to reel in its victim until the target is engulfed. Then, it forms a phagosome, a lipid membrane around the hapless pathogen that holds it helpless. The phagocyte’s lysosomes then fill the phagosome with enzymes and other chemicals that digest the pathogen. The pathogen’s antigen is preserved, however, and part of it is in fact placed on the phagocyte’s surface. Then the phagocyte must find Helper T cells, a type of lymphocyte, and present that enemy antigen to them to start the process of alerting the immune system to the threat. If it helps, you can think of the phagocytes as scouts going out to observe the enemy, then calling in the heavy artillery to finish off their foes. All phagocytes share this basic function, but they do have distinct categories.

The first type of phagocyte is granulocytes. These can be divided into many sub-categories, but we’ll lump them together for now for simplicity’s sake. Granulocytes usually are the primary response to breaches in the skin. While they can perform antigen presentation like other phagocytes, their main job is to consume as many pathogenic organisms as possible. When you have a blister or other infected injury, the pus that it produces (sorry, I know that’s kind of nasty) is composed mostly of granulocytes responding to the infection. They also often specialize to take down larger parasites.

The next type of phagocyte is monocytes. These are fairly basic phagocytes, but they have a trick up their sleeves. When they receive chemical signals indicating an infection in the tissue, they re-organize their cytoskeleton so that they can squeeze between the endothelial cells (the layer of cells that lines blood vessels), and move into the tissue. There, they transform into macrophages (macro: big, phage: eat), huge germ-devouring machines. These killer white blood cells then attack and present antigens to Helper T Cells to respond to the tissue infection.

The last major category of phagocyte is dendritic cells. These perform the same function of devouring pathogens, then activating lymphocytes, but are known for having especially pronounced pseudopodia that they use to reel in foes.They are also capable of filtering harmful substances out of the blood stream.

These are the general categories of phagocytes. The file repository (See Downloads from our home page) has my immune system computer game on it, and a pamphlet with some good info on the immune system (courtesy of nobleprize.org). There are no proper instructions for the game yet so it may be confusing. Feel free to try it, though. Proper instructions and some bugfixes are coming, as well as a few more levels.

Now, I am not going to replace what is already up, because I don’t know who has read it and who has not. However, I would like to add a few things. Here is some info on Lymphocytes:

Lymphocytes
Lymphocytes are the second major category of white blood cells. As you can probably guess from their name, they are found in the lymphatic system. For those of you who are not familiar with the lymphatic system, it is a system of vessels that forms part of the circulatory system. Unlike the blood stream, however, it is full of white blood cells and germ killing chemicals (That’s the complement system: we’ll get to that later). It is also punctuated by lymph nodes, which sort of serve as a headquarters for the immune system’s operations. When a doctor says you have “swollen glands” when you are sick, that is because your lymph nodes are swelling in response to a pathogen.

Anyway, like phagocytes, lymphocytes fall into several categories, each with a very unique function.

The first to become involved, after the phagocytes, are usually Helper T cells. These are the cells that phagocytes present pathogenic antigens to. (As mentioned before, antigens are unique chemical signatures found on the surfaces of cells) When they receive an antigen from a pathogen, they become activated and go off in search of another type of lymphocyte: Basic B lymphocytes.

Basic B lymphocytes (There is probably a more specific name for them, I do not know it) float around in the blood stream and lymphatic system until they come in contact with a pathogen. If the pathogen’s antigen fits the B cell’s receptors, it becomes partially activated. In order to become fully activated, it must then meet up with an activate helper T cell that was activated by the same antigen as the pathogen the B cell encountered. This may sound impossibly difficult among millions of cells rushing about, and indeed it sometimes takes some time for the immune system to go through this sequence of activation. However, once it is complete, the big guns come out.

When a B lymphocyte is fully activated, it begins to go through mitosis very rapidly and as it divides, it produces two main types of cells: Plasma cells and Memory cells. These are the short term and long term solutions, respectively, for an infection. Plasma cells are the immediate solution. They immediately begin producing antibodies. Antibodies are special proteins that can hinder and defeat pathogens in many ways. They are usually Y shaped, but can often appear in the form of many Y’s stuck together. At each of the points of the Y or Y’s there is a site where they can bond to the antigens of the pathogen (the same antigen that started this whole thing). This has varying effects. Sometimes, it is enough to destroy the pathogen, especially if it is hit by many antibodies. Other times, it renders it unable to infect or damage cells, neutralizing the threat. Or, it can bind multiple pathogens together, one on each prong of the Y(s) and then attract phagocytes to gobble them up. Thus, the immune system is usually able to annihilate most foes. A single plasma cell can produce thousands of antibodies very quickly, but once it has done so it usually just dies. However, sometimes foes come back or remain hidden somehow. In these cases, it is up to the memory cells to stop them. These cells do not require a complicated activation process. They simply look for a single type of pathogen, and, once they find it, immediately begin dividing rapidly and signaling other leukocytes to warn them of the danger. In this way, the infection is crushed much more quickly than the first time. This is how people become “immune” to diseases.

There are instances, however, where this does not work. For example, when DNA goes bad, cancer cells appear that can wreak havoc. Viruses and bacteria can enter the body’s cells and infect them, causing damage and hiding from the immune system at the same time. For these cases, the immune system has its elite commandoes: The Killer T cells. These cells are essentially on an eternal search and destroy mission. They move normally through the blood stream, checking on the body’s cells until they find what they are looking for. No virus or bacterium can enter a cell without leaving some of its antigen behind on the surface. Also, cancer cells often release strange chemicals or change their antigens, another sign that can tip off the Killer T cells. When they have a target, the Killer cells apply their weapon: a cytotoxin. A cytotoxin is essentially any chemical that causes cells to cease functioning, and essentially to die. This they apply to infected cells, shutting down the cell and destroying whatever pathogen may be hiding in it, or eliminating the spread of the cancer. This may seem like a bad idea: why kill the body’s own cells? However, when you look at it, it’s really worth the sacrifice. First of all, if the cell is infected or mutated, it is almost guaranteed to die anyway. Second, a single bacterium- or virus-infected cell can create as many as a million more viruses or dozens of bacteria when it dies, and a cancer cell is even worse, multiplying out of control until it creates a tumor. Besides, the body has many trillions of cells, so a few that are destroyed to save millions are not a big loss and quickly replaced.

Anyway, that is my lesson on lymphocytes. If you have questions, please email me or write a comment on the blog page on my profile. Thanks!

Vocabulary:
Okay, so putting formal definitions in dictionary format was a really bad idea. On the subject of vocabulary, you are probably already familiar with Virus, Bacterium, Fungus, White blood cell, Red blood cell, capillary, vein, artery, and tissue. I will also assume that you are familiar with the common organelles found in a cell and the categories of biological compounds (lipid, protein, carbohydrate, etcetera). Check out Kevin's biology page if you are unclear on any of the above. Here are some definitions that you may not know that will come up in this course:

Phagocyte : A type of white blood cell capable of engulfing and destroying pathogens. (Phago: eat, Cyte: cell)

Leukocyte: Scientific synonym for white blood cell (Leukos: white, cyte: cell)

Pathogen: Any harmful microorganism

Phagocytosis: The process of a phagocyte ingesting a pathogen

Antigen: A chemical signature present on the membranes of all cells. These are unique to cell types

Phagosome: A pocket of space surrounded by a membrane formed inside a phagocyte where an engulfed pathogen is held before being destroyed

Pseudopod (Plural: Pseudopodia): A protrusion or bulge in the cell membrane intentionally created by the cell, often used for movement or, in the case of a phagocyte, to grab pathogens. (Pseudo: fake or temporary, Podia: feet)

Lymphocyte : A type of white blood cell that originate in the lymphatic system and respond to antigens received from phagocytes and pathogens.

Antibody: A type of protein that can attach to the antigens of pathogens in order to hinder their reproduction, eliminate capability to cause harm, mark them for destruction by phagocytes, or just plain kill them.

The Lymphatic System : A system of vessels that forms part of the circulatory system, but is filled mainly with white blood cells and various chemicals.

Complement System: A classification including various proteins and chemicals produced by the body to hinder and destroy pathogens.

Participation
If you would like to participate in this class, please click the edit button to edit this page and type your name in at the end of this section. I will not send you files and update notices unless you are signed up here. If you would like me to send you things at an email address other than your wildwood one, please include that as well. I will not update this page until people are signed up here so I know you have read it. I have deleted the names of anyone who signed up last year, so please sign up again if you would like to continue taking the class. The class will actively run for approximately 4 months, at which point it will be archived and I will start another class. Anyone who signs up here will be expected to keep up with all material in this class until the end of the class. There will be no tests and probably no homework except to read this page and anything else I send you. Thanks!