 Hey everybody, Dr. O here. So in this video I want to cover the primary versus secondary immune response. I also want to talk about how that influences immunity and how that influences vaccination as well. So the primary response is the first time you're exposed to a new or novel antigen. The secondary response would be or should be every time after that. So let's go ahead and look at the two and then we'll see where vaccines fit in here. So the first time you're exposed to a pathogen, you usually get sick. I mean you don't technically have to, but you usually get sick and then once you've been exposed to something you should now be immune and not get sick when you're exposed to it again. So let's go ahead and look at why. It has to do with the antibodies. So here we see on the left hand side the primary immune response. Notice that the first thing that happens is you get an increase in IgM antibodies. That's totally fine, but it doesn't go very high and IgM antibodies are relatively weak compared to IgG antibodies. Notice how long it can take 10, 14 or more days to mount a complete IgG immune response. The first time you're exposed to something. So you'll notice here that IgM show up first, then there's a delay and then IgG antibodies show up later and the response is not very powerful. As you can see that red line doesn't go very high and it's also not very sustained so that the line will drop back to normal as well. So that's why the first time you get exposed to something, your chances are you're going to get sick. You don't have a full immune response to it. Now on the right hand side, the secondary immune response is going to be much more powerful. Yes, the IgM's still show up, but they show up earlier as well, but the IgGs are the key. They just show up on the scene super, super quick so you don't have to wait near as long. The response is much more powerful and look how sustained it is as well. So you get a quicker, more powerful and sustained immune response, which is why once you've been exposed to something you should now be immune to it. So the second time you're exposed to something, your immune system should just wipe it out before you even know that it's there, to be honest. So that is the primary versus secondary immune response and that's what leads to immunity. It's why if you've been exposed to something you should now be immune to it. So of course there are a lot of caveats there. What you've been exposed to could change or like for example like why, okay, why do I get the cold every year if I should not be immune to the cold? Well, there are thousands of viruses and variants of viruses that can lead to the cold or same thing with the flu. The flu is constantly changing. So yes, if you've got the flu before, you can get it again because it's a new, it's a new flu, it's a new version of the flu, things like that. So this is not completely foolproof but that is the difference between the primary and secondary immune response. So how do I use this, use this image to help someone understand what vaccines do? So vaccines will actually, so you see primary antigen exposure. Vaccines will expose you to antigens that have been weakened or killed or you're just exposed to subunit vaccines that only should expose you to parts of an organism, you're still going to be exposed to antigens which means your immune system will see those antigens and mount an immune response to it but you never got exposed to the actual pathogen and especially at full force. So how I describe how vaccines work is the first time you come into contact with a pathogen in the world is the second time that your immune system has seen it. So when you get exposed to diphtheria, if you've been vaccinated, the secondary immune response is going to be what pops up even though it's the first time you've ever bumped into that pathogen. So that's how I like to tell people how vaccines work. They bypass this primary immune response where you usually get sick and some people die so that the first time you're exposed to something is the second time your immune system has seen it. Okay, so that is the primary versus secondary immune response. I hope that helps. Have a wonderful day, be blessed.