For the T helper lymphocytes, what exactly are their functions? When you look at the word helper, what does that word helper imply exactly? What are they helping, right? As with all lymphocytes, even the T lymphocytes start off as dormant, which means to say they are inactive. So how do they become active at first?
So in a little bit of revision, if you want the T helper cells to become active, they are activated because of macrophages. And remember, we talked about macrophage a few videos ago, where I said macrophages are phagocytes. where they carry out phagocytosis, engulf, ingest, and digest the pathogens, but they will also present the antigen on their cell surface membrane. So as an example here, as you can see, let's say a pathogen enters your body.
body and starts to cause an infection, the pathogen starts reproducing or replicating in your body, one of the macrophage that comes in contact with the pathogen will engulf it, ingest it, and digest it using lysosomes. But it will also present the antigen on its cell surface membrane. And if you notice, there is also a lot of pathogens present in the vicinity because the macrophage cannot be removed. Perhaps the macrophage may have difficulty engulfing all the pathogens, right?
So we have to find another way to eliminate all the other pathogens in the area. And what the macrophage does is it will then present the antigen to the T helper. lymphocytes. So look at the first T helper lymphocyte.
Is the receptor complementary to the antigen? No, it is not. And as you can see here, one of the T lymphocyte that was dormant will eventually bind with the antigen.
And when it binds with the antigen, the shape of the receptor and the shape of the antigen is complementary. This process is known as lymphocyte. clonal selection of T lymphocytes. So the T lymphocytes in most cases are usually stimulated by the macrophage. The macrophage will present the antigens to the T lymphocytes.
So in this case, the T helper lymphocytes with complementary receptors are stimulated and same with B lymphocytes and T killer lymphocytes. Once they undergo clonal selection, they will definitely undergo clonal expansion. And when they undergo clonal expansion, they will undergo mitosis repeatedly and they will form memory T-helper lymphocytes.
I'm not going to focus on that yet, but they will also produce activated T-helper lymphocytes. Now, if you remember in the previous video, activated T-killer lymphocytes will kill off infected cells. So what do activated T-helper lymphocytes actually do? what are they helping?
The activated T helper lymphocytes will release a chemical or a group of chemicals known as cytokines. So you might be thinking, what exactly are these cytokines? And they're just a type of signaling molecule, all right? So when you see the word signaling molecule, it just means it's supposed to tell another cell to do something.
In Chapter 4, I told you before that in cell signaling, a cell may release a signaling molecule. It may bind to another cell's receptor and produce a response. The cell will produce a response. A cytokine is an example of a signaling molecule. So what exactly is the cytokine going to do?
You see, in your immune system, what we have studied so far is we have studied T helper cells. Well, we are studying. T helper cells right now, we have studied B lymphocytes, T killer lymphocytes, and also some macrophages. When the activated T helper cells release cytokines, some of the cytokines will travel in the body and they will stimulate the B lymphocytes, but they will not stimulate all the B lymphocytes. They will stimulate the appropriate B lymphocytes as well.
The ones with the receptors, which are complementary to the antigens attacking our body. So when that, see that means it only stimulates the one that I've highlighted like that. And when that B lymphocyte is stimulated, guess what? It will immediately undergo clonal expansion. So you see, that B lymphocyte did not have to directly meet the pathogen.
or it did not directly have to meet the antigen. It was stimulated by the cytokines that came from the T helper cells. So when that B lymphocyte is stimulated, it will produce memory B lymphocytes and plasma cells, and those plasma cells will secrete the complementary antibodies which may bind to the antigens, which are the antigens which are causing problems in your body. So this means that the B lymphocytes can be stimulated remotely by the T helper cells.
And that's a good thing, right? So the cytokines can also activate T killer lymphocytes. Remember, the T killer lymphocytes were dormant. But does it activate all the T killer lymphocytes?
No, it doesn't. It only activates the appropriate T killer lymphocyte and stimulate them to undergo clonal expansion as well. And When they undergo clonal expansion, they'll produce the activated T-killer lymphocytes, which will go and destroy any cells in your body which are infected. The cytokines will also stimulate macrophages, and it will stimulate the macrophages to increase the rate of phagocytosis. So in a way, the T-helper cell is sort of like a coordinator of the immune system.
It tells other white blood cells what. to do. So like for example, as you can see here, the antigen is presented and when the antigen is presented by the macrophage to the T helper lymphocyte, the T helper lymphocyte is activated and it will release cytokines and the cytokines will stimulate the B lymphocytes and it tells the B lymphocyte, hey, we are being infected by this pathogen. You need to produce this particular type of antibody to attack.
that pathogen. It will tell the T killer cells, look, some of the pathogens have infected the cells. Your job is to go look around the tissues to destroy any infected cells. And it also stimulates the macrophage to increase the rate of phagocytosis and destroy and digest all the pathogens present in the body.
Some of my students will ask this pretty interesting question. Why exactly oxygen? are T helper cells extremely important? Because remember, all they do is just stimulate the B lymphocytes, T killer lymphocytes, and also macrophages.
And if you remember, I'm just showing you here the T helper cells. You can see some T help. you can see the T helper lymphocyte, which is inactive, a B lymphocyte, which is inactive, a T killer lymphocyte, which is inactive, and a fried egg. Every time I draw the macrophage, it always reminds me of a fried egg.
I don't know why. It kind of looks a bit like it. So you can see the four types of white blood cells that we have studied so far.
When you studied B lymphocytes, they can be stimulated directly by pathogens. through a process known as clonal selection. The same thing can be applied for T-killer lymphocytes. They can be stimulated by infected cells carrying out antigen presentation.
And macrophages can also be stimulated directly by pathogens. So if each cell can be stimulated directly by pathogens or antigens, why in the world do we actually need T helper cells then? Well, the thing that we have to understand, if you remember, just a bit of quick revision, when the T helper cells are stimulated through antigen presentation by macrophages, they will undergo clonal expansion. to produce memory T helper lymphocytes at the top and activated T helper lymphocytes. And those activated T helper lymphocytes will release these cytokines, which are signaling molecules.
What the cytokines will do is they'll stimulate the B lymphocytes, T killer lymphocytes and macrophages remotely. And I told you that it coordinates the immune system. The beautiful thing that T helper cells do is they can simultaneously stimulate many immune cells at the same time.
Because if you look at these cells over here, if the B lymphocyte is stimulated, the B lymphocyte cannot tell the T killer lymphocyte what to do. And the T killer lymphocyte, if they get stimulated, they cannot tell the B lymphocytes or macrophages what to do. They cannot communicate with each other.
But the beauty is the T helper cells with the help of cytokines can tell all the three different types of white blood cells at the bottom, look, we are being infected, you need to do something about this. That is why they are known as the coordinator of the immune system. immune system.
That is why T helper lymphocytes are imperative. They are important in our immune system. So what if there are no T helper lymphocytes? Remember when we studied HIV and AIDS, I told you that the HIV pathogen can destroy the T helper lymphocytes.
I want you to understand this. They only destroy the T helper lymphocytes. Do they destroy the B lymphocytes, T killer lymphocytes? and macrophage? No, they don't.
These three white blood cells are still intact in your body, by the way. But here's the problem. If different types of pathogens try to come into your body and proliferate and replicate, these pathogens can start causing an opportunity. infection.
Now you might be thinking, wait, we still have other white blood cells. Why don't these other white blood cells go and attack the pathogen? Well, for the very simple reason that there are no T helper lymphocytes to release cytokines.
And when there are no release of cytokines, it actually slows down your immune system. And when it slows down your immune system, it leads to a condition known as AIDS, where you have a lot of different types of opportunistic infections happening in your body. That's the important thing you have to understand about HIV AIDS.
When a person has AIDS, their immune system is technically crippled, not because all the white blood cells are gone. The only white blood cells that are gone are the T helper lymphocytes. Your B lymphocytes, T killer lymphocytes, and macrophages are still in, but but they slow down.
Why do they slow down? Well, because there are no cytokines released by the T helper cells. So some students will ask the question, wait a second, I thought we have memory cells, memory B lymphocytes, memory T killer lymphocytes and such.
Don't they produce a faster response to the pathogen? They do, but for them to function efficiently, they also need cytokines. That is why cytokines from the T helper cells are extremely important.
And that is also why when HIV destroys the T helper cells, the entire immune system will shut down almost completely.