 HIV or Human Immunodeficiency Virus. Do you know, we got this virus from the chimps? Yes, just like the Corona virus that we have got from the bats, it is believed that somewhere in the mid 19th century, this virus must have jumped from the chimps into us humans. But it was only in 1983 that this virus was finally isolated, identified and was properly studied. And since then, according to reports, it has taken over 37 million lives. Imagine how deadly this virus is. And it is able to do that because it affects the immune system. As the name goes, Human Immunodeficiency, it causes some deficiency malfunctioning in the immune system itself. Let me bring in the HIV virus in the circulation. So here is the HIV virus in the human circulation. And once it is inside the circulation, it will come across a variety of different cells, right? So these are the different type of cells. Now, how does the virus decide which cell to go and infect? Now we already have discussed that it goes and infects the immune cells. But how does it find it out that this particular cell is an immune cell or not? So it do that by a very special type of protein that it has on the cell surface or on the surface of the virus, which is called GP120. It is glycoprotein 120. And this glycoprotein 120 goes and binds to a very special type of protein, which is called CD4, which is cluster of differentiation for. And unfortunately, the CD4 is present on the immune cells. Let's say this is CD4 protein. Now this HIV virus as it is moving through the circulation will come across different type of cells. And as soon as it finds the one that has CD4, boom, the outer membrane of the virus and the outer membrane of the cell, it fuses and the virus release all its content inside the cell. And what happens after that? This HIV virus hijacks the entire cell and gradually the entire human body. So here we have an HIV virus. And as we discussed the outermost protein of HIV virus, this one here is called GP120. So this is a glycoprotein 120, a protein that binds to the CD4 of a cell. And along with that, we can see other parts also, right? The outer part, this one is a membrane, which is exactly the same of human cell membrane. And later we will know that this virus actually stills the outer covering of the cell of humans. Okay. And inside the cell membrane, you can see this yellow thing. This is called the cap seed or the envelope or you can see a bag that carries the genetic material. Okay. Now what is the genetic material of HIV virus? RNAs. So let me write it here. It has two single-stranded RNAs. And along with that, can you see this three dots here? These are actually enzymes. It also carries three very important enzymes with it. And I like to call them RIP. You must be thinking that the HIV virus must have infected the person. It's time for him to rest in peace. Well, it's actually not that. I'll tell you their names and why do I call them RIP as we come across their functions in the video. Okay. Okay. Now let's quickly see what happens when this HIV virus fuses with an immune cell. Let's say with a macrophage. Generally any human cell is oval or circular, but I have made a rectangular human cell so that I get more space to write. Okay. Now as you can see, after the fusion, this cap seed has dissolved and this HIV has released its content into the cell. Now for any virus, whether it be an RNA virus or DNA virus, its main goal is to make more and more protein, viral protein inside the cell so that they can make more viruses. Now in a human cell, let's discuss how we make proteins. In a normal human cell from DNA, from DNA, we make mRNAs. And from mRNAs, we get proteins, right? But here in case of this virus, you can see it has come inside with RNAs. Now, what do you think will the virus do? What strategy will it apply so that it can get a continuous flow of viral proteins? Okay. Let me tell you, the virus makes viral DNA and incorporate that DNA into the cell's DNA so that when the cell will replicate its DNA, it will end up replicating the viral DNA as well and then make viral proteins. Okay. That's a nice strategy. But how will the virus make DNA from RNA? Because this is something unusual. This never happens inside the cell. And also our cell do not have any enzymes or any tool that can convert an RNA into a DNA. So how will the virus do that? Well, the virus knew already that it has to do that and therefore it carries the weapon along with it. And what is that weapon to convert RNA to DNA? It is an enzyme called reverse transcriptase. Okay. Now you can probably guess where this R in RIP comes from. This R stands for reverse transcriptase. It is a very interesting enzyme and has dual function. Now let me quickly tell you what these functions are. So its first job is to make a strand, a DNA strand, complementary to the viral RNA. Well, the same thing happens for both the RNAs. But for the ease of explanation, I'll just show one RNA. Okay. But the same thing is happening in both the RNAs. Now, after the complementary strand of DNA is made, this reverse transcriptase does its second job, that is to dissolve this piece of RNA and then make another strand of DNA complementary to the previous strand. And that way the RNA virus makes a double-stranded DNA with the help of reverse transcriptase. So this is a very, very important enzyme. After this, the next plan of this HIV virus is to integrate this double-stranded DNA they have made into the cell's DNA. They now plan to enter into the nucleus. So let me quickly make the nucleus of this cell. So this is the nucleus of the macrophage or the immune cell that has the DNA. And again, I have made this nuclear membrane square on purpose so that I get more space to draw. Okay. Now this double-stranded DNA will slowly start moving towards the nucleus and this time it will take another enzyme along with it. And now the friend whose name is integrase. So now you know where this I comes from. I stands for integrase. And what this enzyme does, you might have probably guessed, it also has dual function. And the first function of this integrase enzyme is to make a cut in the human DNA. So it makes a cut and breaks the human DNA into two. Now its second function is to stitch the viral DNA with the human DNA. So its first function is to cut and the second function you can say is to paste the viral DNA into the human DNA. Now the major job is done incorporating the viral DNA into the human DNA. Now anytime the cell is stimulated to replicate to produce its own protein, it will end up producing viral mRNA and thus producing viral proteins. Now this viral protein has everything required for a virus to form and also to survive. It has parts that will make up the capsid. It has parts that will make up the RNA. But the problem is this viral protein is made into a single stretch of protein and it has to be cut into smaller smaller pieces or individual pieces so that they can properly assemble to form from proper viruses. Well obviously we do not want that to happen but guess what? This virus is not listening to us. It has its own tools. It has come with another friend called protease that cuts this viral protein into smaller smaller individual pieces. And that is where the P comes from. P stands for protease. So this protease cuts this viral protein into individual pieces so that they can properly assemble into new viruses. Okay now let me show you how this virus is but out from the immune cells. Now once the protein is cut into individual pieces the protein which is supposed to be on the outer layer like the GP120 it slowly moves towards the cell surface and it pops out of the cell membrane. Okay and then it slowly forms a curvature there where other parts required to form a virus aggregates together and slowly it buds out from the immune cell. And this is how the HIV virus replicates within the human cell and as you can see as it buds out it pinches a part of the cell membrane of the immune cell and that is how the cell membrane of HIV virus and of us humans are exactly same. But now the question is all this while what was our immune system doing? Normally when a cell is infected with a virus or any other pathogen it displays a part of that parasite or that invader to the outside world or you could say to the immune system with the help of MHC molecule. MHC class 1 molecule. This is nothing but another protein which cell displays on its surface so that immune system can recognize and kill this cell. So that along with it the invader will also be killed. But do you know what the HIV does when it enters the cell? It produces some viral protein which is so potent that it do not even allow the cell to produce this MHC molecule. So sad you see not many pathogen or viruses are able to do that to us. So this HIV is not letting the MHC molecule to be produced. So even though this immune cells are patrolling every part of our body in search of invaders it will have no clue that this particular cell is infected. And not just the MHC molecule this HIV virus also pulls inside the CD4 receptor that's on the cell surface. Well if you can recall this was the receptor initially that helped this HIV virus to refuse with this cell right? Now why on earth will this HIV virus pull this inside? This is because like MHC the CD4 receptor also has the power to present antigens or pathogen that has invaded the cell. So it also has the power to display part of the virus to the immune cell and therefore this HIV virus also takes in the CD4 receptor and this HIV virus doesn't stop there. There are other proteins inside the cell the cells own protein that can actually destroy the HIV's genetic material but by the time those proteins get activated and start acting the virus has already started producing its own protein and those proteins do not allow the cells protein to act on the virus. So this virus takes multiple measures to keep it safe and to keep generations of virus going and our immune cells only comes into play when the viral load in the body increases when there are too many viruses in the fluid where they can easily be detected and killed by the immune system but by then the virus load becomes too high for our immune system to fight and that is how by harming our immune cells it lowers the immune power and also its power to replicate in millions and just very very less time makes it a very important virus.