 Covishield vaccine is the product of the University of Oxford and company AstraZeneca. For understanding how Covishield acts, we need to be a little bit aware about the structure of coronavirus. So this is the structure of coronavirus. This virus on its coat expresses proteins known as spike proteins and it is via these spike proteins that this coronavirus attaches to cells and enter into the cells. So basically the mechanism of action of all the vaccines which are being developed is basically to prevent the attachment of the coronavirus to the cells. So that at beginning only the attack of the virus can be prevented. But what is the mechanism by which Covishield prevents this? Basically Covishield uses another virus that is adenovirus. This is a harmless virus and mostly causes a common flu, the common cold which we routinely experience. So it is a modified adenovirus which has lost its ability to multiply in the body. So it is derived from chimpanzees and even though it can enter cells, it cannot multiply in the cells. Now what AstraZeneca did is that this genome of the coronavirus has a part which produces these spike proteins. So this part of the genome of coronavirus is integrated with the genome of the adenovirus. So if this adenovirus goes into the cells of our body, then these spike proteins can be produced in the body itself. So once this adenovirus is inserted in the form of the vaccine into the body, this adenovirus goes and attaches to the cells and the cells actually take up this virus inside. So this goes inside into the cell, it moves out from there and inserts genome into the nucleus of our cells. So as you might be aware that basically all the virus use our body's cell machinery that is the nuclear machinery to produce the proteins. So once this DNA is inserted into the nucleus, this DNA is transcribed into messenger RNA and from these messenger RNA ribosomes produce proteins. So the spike proteins are produced by these cells. Now once these spike proteins are produced, some of these start appearing on the surface of the cell. So cell expresses them on their surface, it's like telling others that there is this foreign attack on me. So the foreign antigen starts appearing on the surface of the cell. But remember that this adenovirus is not doing anything, it is not even replicating, it is not dividing in the cell, it is only the protein, the coronavirus protein. Now it is being expressed. So this adenovirus is acting as a vehicle which is taking this coronavirus genome and the cell is able to produce these spike proteins. So once these spike proteins are expressed on the surface of the cell, now two things happen. One, the cells express the protein on the surface and secondly, once these cells die, once these cells die, they release these proteins into the environment. So these released proteins are then recognized by other cells of our body which are macrophages and these macrophages basically engulf the fragments of these antigens that is the spike protein. So then what these macrophages do, that they present these small fragments to other cells. So that is why they are also known as antigen presenting cell. They are presenting the antigen to other cells. So what are these cells? Basically it is helper T cell. So basically it is a signal to the helper T cells that this kind of foreign antigen has come. So you have to be active against it. So one is helper T cell is getting signal from antigen presenting cell. Then there are other cells also that is B cell. So B cells what they do is they get signal not only from the antigen presenting cells but as I told you that the cells which were expressing their spike proteins on their surface because of the adenovirus they also send the signals to the B cells. Now these helper T cells as the name suggests that it is helper T cell. They help the B cells. So one they are getting the signal of the antigen plus the helper T cell which is also recognizing the same antigen it sends the signals to the B cells and activates them further. So these B cells now start maturing and proliferating causing the formation of the plasma cells which ultimately secret antibodies against these spike proteins. So this is one mechanism that vaccine by delivering the spike protein through adenovirus is causing the activation of the B cells and the release of the antibodies from the B cells. Now this antigen presenting cell also presents this antigen to killer T cells. So presentation of antigen to helper T cells, B cells and also to killer T cells. So as the name suggests killer T cells they can actually kill the infected cells. So they also get signal and these helper T cells which have activated the B cells they also activate the killer T cells. Basically these are really certain chemicals known as cytokines which activate these B cells and killer T cells. Now body has a mechanism that once these B cells and T cells identify a particular antigen they create a memory of it. So there are memory B cells and memory T cells such that if there is a exposure to the antigen next time another time. So these memory cells can respond faster. So what happens as you might be aware that there are generally multiple doses of vaccines required in case of COVID shield. Second dose is required around 8 to 12 weeks after the first dose. So with the first dose what happens if you see this graph where x-axis is showing the time in weeks and y-axis is showing concentration of antibodies. With the first dose the response starts after almost one week and then it increases. See the concentration of antibodies which are being released by B cells increase and then it starts coming down. So around 4 to 6 weeks you see it has started coming down and that is the time actually when the second dose of the vaccine is given. Now because as already told that the cells have generated memory cells with the second dose there is an immediate response. So concentration of antibody rises very fast. If you see the first response took one week but the second response has taken very less time and the concentration of antibodies has risen very fast and that too the response is much higher. The concentration of antibodies has increased too much plus the antibodies remain in body for a long duration maybe up to the months. So we are saying that we are not very sure that how long this covishield effect will last. So people are still gathering data about that and maybe after some months another dose of covishield may be required to keep this concentration of antibodies higher. So this is known as a secondary response which is occurring to the second dose of the vaccine with the one which has occurred due to the first dose is the primary response. Okay so now if coronavirus enters into the body then what will be the response of the body. So once the coronavirus enters actually the antibodies if they are already present in the body because of the secondary response then these antibodies at the first sight of the virus will actually go and attach to the spike proteins which they had already recognized before. So they will go and attach to the spike proteins and prevent the virus from attaching to the cell. So this is known as neutralization. This method of acting of the antibodies is known as neutralization where they go and bind with the active component of the pathogen and prevent it from entering into the cell. So already circulating antibodies neutralize the spike proteins. Apart from this whenever a pathogen enters it is taken up by our macrophages which are the antigen presenting cells and they again present this antigen to B cells killer T cells and helper T cells and because of the vaccine since these cells have a memory. So obviously the response to the pathogen this time is much higher and the activation by T helper cells is also much more for the B cells and killer T cells. So obviously more antibodies will be produced the response of the body will be more plus if the virus manages to enter into the cells what happens that these cells start expressing certain proteins and trying to signal to cells that okay I am infected. So these infected cells are then killed by our T cells. So our B cells are preventing the attachment of the virus to the cells but if it does manage then this infected cell is killed by the killer T cell. So friends that's the mechanism of action of the vaccine CoV shield. Hope you have understood the background immune mechanisms of our body which are activated due to the vaccine and how it helps us to effectively attack the virus which may enter in our body in some future time. Well thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel Physiology Open. Thank you.