 We are talking about immunology. At some point or another, we all get little cuts and bruises and sometimes you get pricked by thorns or wood splinters. We have experienced these but we don't know what happens at the cellular level. Let me show you what happens at the cellular level when we get a prick from a thorn or a wood splinter. We just talked about scenarios, war scenarios that the first requirement for defensive forces to mount a response to an attack is detection and announcement of that detection. In our body, injury is detected by specialized cells which are present in most tissues. These cells are called the mass cells. I'll point them out right here. These cells are full of vesicles. These vesicles contain special signaling molecules that can alert the rest of the body that there is an injury has been detected. Mass cells drive their name from the German word mastos, meaning nourishment, because when they were discovered first by a German scientist, he saw these cells and the vesicles, these cells full of vesicles, he thought that these cells were basically responsible for providing nourishment for the surrounding cells. That's not their function, of course. These vesicles contain a signaling molecule, one of them being histamine. When histamine is released in the surrounding tissue, it does several things. One of the things it does is it dilates the blood vessels or the capillaries. These vesicles, they're of course lined with cells. When they're stretched, they become leaky and material can go in or it can come out of the injury zone. So injury has been detected. It has been announced. The first light infantry, which I refer to as cells that are going to come in and they're going to try to contain the problem. Those cells are basically the phagocytic cells shown here. These cells, they are attracted to histamine and other signaling molecules that are being released at the site of injury. They find that injury, they will move in and they will see if there are any pathogens at the site of injury. If there are, for example, bacteria present, which are shown here in purple, if they encounter these bacteria, they will engulf them. They will phagocytose them. They will ingest them and contain them. So these bacteria do not go to other places and cause infection. So these phagocytic cells move in and contain the infection. Another thing these phagocytic cells do is they release interleukin-1. Interleukin-1 is also a signaling molecule. It interacts with the thermostat in our body. As you know, we geysers or heating devices have thermostats. These instruments basically regulate the temperature. So interleukin-1 can signal our thermostat to increase the body temperature. Remember, I told you that our enzymes are more resistant to higher temperature than the bacterial enzymes. So these cells are containing the infection. They are signaling the thermostat in our brain to increase the temperature. So these bacteria will not be able to spread and grow. So this is the first response. So when that happens, after the injury has been detected, other cells have also come to the site of injury and they do their job. The cells that are making the lining of epithelium or the blood vessels, they start expressing special proteins on their surface. And these proteins, some of these proteins initially first of all slow down the phagocytic cells or the white blood cells or in other words the cells that are going to fight the infection it causes them to slow down their movement. They're floating in the bloodstream. When they detect these special signaling molecules on the surface of certain cells, they slow down and they start rolling till they come close to the site of infection or the injury. At that site, this flatten out, as you can see in this diagram, the cell is flattening out. We will also see this in an animation. The cell is going to flatten out and it is going to crawl between the space between the two cells. The cell is going to go through these cells in between these cells and go at the site of infection. So here is a way how cells are attracted to the site of injury. Soon after that, the histamine is signaling seizes. Other parts of immune system that we will talk about in much later, the complement system, it can also cross through those gaps that I talked about when the blood vessels are leaky. The purpose of this exercise is to contain and neutralize the pathogen that is invading the host territory. So this is basically an exercise initially to first of all limit the scope of infection. So by first of all engulfing these phagocytic cells, engulfing the bacteria or the pathogens and also signaling the thermostat in our brain to increase the temperature a few degrees centigrade to restrict the ability of these bacteria to divide and survive in our bodies.