 Hey everybody, Dr. O'Hare. In this video I want to do an introduction or an overview to the idea of the mechanisms of pathogenicity. So in previous chapters we've talked about how organisms spread from one person to another, from one host to another, from one surface to another. Now we're talking about they found their way at an individual human being. How are they going to become a pathogen? How are they going to cause a disease? So like I said, I will cover some of these concepts in quite a bit more detail in further videos, but I want to make sure you have all of the basic steps here in one place. So let's start with number one. At the beginning, they have to find a way in or on you if they're a skin infection, but a portal of entry. So the most common portal of entry is going to be our mucus membranes, lining all the cavities and passageways into and out of our body. So the most common portal of entry of those is going to be the respiratory system, then we have the digestive system and the genitourinary system. So those are going to be by far the most common ways in. Every time we eat, every time we breathe, every time we do any of these things, we're potentially being exposed to pathogens. So they have to find a way in, but there are other ways where it's skin. So the skin infections, obviously all you have to do is get into the skin and then you can even, there are some organisms that can get through and worm their way through intact skin. The third major portal of entry is called the parentarial route when you bypass these defenses. So bypassing the skin, bypassing mucus membranes, I generally think of like biting insects, needle sticks, bites from animals like dogs and those kind of things would be some of the most common parentarial routes of entry. All right, so now the organism has found its way in. The next two things that have to happen is number one, there has to be enough of them. So we will cover this idea of the infectious dose or lethal dose. I'll use terms like ID 50 or LD 50. So infectious dose ID 50 basically means how many organisms would it take to make 50% of a population sick? The lethal dose is usually talking about toxins, how much of a toxin would have to be present in order to potentially make someone's very sick or kill them. So found a way in, there has to be enough of them. The next key step is adherence. That's why I always say that adherence is really the beginning of an infection because if an organism can't find a home, can't find the right location, can't colonize you, then it can't make you sick. So portal of entry, do you have enough of the microbes or enough of the toxin and then adherence? Next we go into this organism now has to worry about the immune system. So either penetration or evasion of host defenses. So we'll look at things like structural things that make it so, a bacteria for example, can't be engulfed by phagocytes. So capsules, some of the proteins on fimbria can do this. These types of things or evasion of the immune system. Some organisms actually hide inside of our cells. So they would be called intracellular pathogens. Others can hide in plain sight because of a process called antigenic variation where they change what their surfaces look like. So the immune system is looking for an older version of an organism than what's still available. So again, I'll cover all these in detail, but just understand that once an organism is found a way in, there's enough of them. They found a location, they found a home. They have to evade the immune system or destroy our immune system long enough to reach a population that's actually ready to do the damage. So once you've done all these things, now we have a population of organisms that's ready to strike, that's ready to do their damage. So damage to host cells, which would be, we'd be the host. There's just direct physical damage. You've got organisms that produce enzymes that basically just chew up our body proteins. You've got organisms that are going to steal our nutrients. A common example there is the citrophores, these chemicals that are produced that will steal our iron. So there's kind of a war for iron between us and some pathogens. The big one that we're going to focus on is the toxins. So we will talk about endotoxins and exotoxins and where they come from and the kind of damage they do. So the organism is found a way in. There's enough of it. They had adherence. They found a location. They were able to hide or penetrate or damage the immune system long enough to reach the numbers they needed to strike. And now they've struck, I guess, yeah, they've struck. So they've damaged us. They've damaged our cells. The last thing they need to do is find a way out and that's going to be a portal of exit. They don't want us to be a dead end. They want to spread their offspring throughout the world and find other hosts. So the portal of exit would be its way out. The main thing to note here is that the portal of exit is usually the same as the portal of entry. So if you like to come in through the respiratory system, that's how you're going to leave. So the portals of exit and portals of entry are basically the same. Now once an organism finds its way out of you, its goal is to find a portal of entry in another host, in another human in our case. All right. So like I said, I have to go through a lot more detail of some of these topics, but I wanted you to have all of them in one place, this process from start to finish. So these are the general mechanism of pathogenicity. So I hope this helps you. I look forward to seeing you in other videos where I do dive a little deeper. Have a wonderful day. Be blessed.