This video, we're going to begin talking about monocytes and the cells that derive from monocytes. Taking a look at our map of the lesson down below on the left-hand side, notice that we're focusing specifically on these monocytes and the cells that are derived from them, including the macrophages and the dendritic cells. These monocytes are capable of developing into one of 2 main types of phagocytic cells or cells that are capable of performing phagocytosis or cellular eating. These two main types of cells are the macrophages and the dendritic cells. The macrophages are, once again, phagocytic cells, meaning that they're capable of performing phagocytosis. They also serve as sentinel cells, which we call lookout or guard cells that act as scouts. These macrophages are capable of responding directly to an infection to help eliminate it but can also alert other host defenses as well, and that is really what makes them sentinel cells. There are many different types of macrophages found in different types of tissues, and we refer to these macrophages as resident macrophages. Resident macrophages, as their name implies, reside permanently in specific tissues, which means that they remain stationary in the tissues, and they don't really migrate around to other tissues. They reside in the tissues. These resident macrophages, because they reside in the tissues, will already be present in the tissues whenever those tissues get damaged and microbes invade those tissues. They'll already be present and ready to respond because they reside permanently in those specific tissues. Other macrophages we refer to as wandering macrophages, and as their name implies, they wander around or move around through the blood to other sites of infection. These wandering macrophages can be recruited to a site of infection. Again, they travel around or wander around through the blood, circulate through the blood, and they can leave the bloodstream to a site of infection when they are recruited.

Now, giant cells refer specifically to macrophages that have fused together, and the fusion of macrophages creates these giant cells. The giant cells, because they consist of fused macrophages, have increased destructive abilities. If we take a look at our image down below on the left-hand side, we're specifically focusing on the macrophages. Here we're showing you an image of the tissues, the skin here. Notice that the skin here has been damaged, that there's some kind of damage here, and there's some kind of sharp object that has penetrated into the skin, and microbes are now infecting this open wound. Notice that there are already some macrophages that are present in the tissues already, and we refer to these as resident macrophages because they reside. They are living in the tissues, and they're already present and ready to respond whenever any of our tissues get infected. Wandering macrophages, on the other hand, circulate through the bloodstream. As they circulate through the bloodstream, they can be recruited to a site of infection. The resident macrophages reside and remain stationary within a specific type of tissue, whereas the wandering macrophage can circulate through the blood and move to other locations and be recruited to sites of infection.

The second main type of cells that monocytes can develop into are going to be the dendritic cells. Dendritic cells also serve as phagocytes, but they are mainly going to be important as sentinel cells, which again are lookout or guard cells that act as scouts. Their job is to sense and detect the signs of microbes, and then they can alert other immune system cells and activate other immune system cells. These dendritic cells are sentinel cells with long appendages. They reside in tissues, and they are capable of alerting and initiating adaptive immunity and adaptive immune responses. They can alert and activate B cells and T cells. We'll be able to talk more about this process of dendritic cells alerting and initiating adaptive immunity later in our course when we're talking more about adaptive immunity. Ultimately here, what we're saying is that these dendritic cells can ingest pathogens in the tissues. They can ingest those pathogens, break them down, and then present pieces of those pathogens to cells of the adaptive immune system in order to activate those adaptive immune system cells. Once again, we'll get to talk more about this later in our course, but down below we're showing you a little image of these dendritic cells. These dendritic cells have these long appendages, as you see here, that project out. Here what we're showing you is a micrograph of what these dendritic cells can look like under a microscope. Notice that they have these long appendages that stick out. Here we're showing you the intestinal lumen. This is our intestines, for example, our large intestine. Some of the substances that we end up eating get broken down into antigens, and these antigens could be potentially from microbes that we have eaten. What you can see here is that the dendritic cells, which are down below, have these long appendages that can take little samples of the antigens in our intestine, and then they can ingest those antigens, break them down and process them, and then present them on their surface. So, notice that these little pieces of antigen are presented on their surface, and here's a second dendritic cell doing something similar. Then these dendritic cells can go on to activate other adaptive immune system cells like B cells and T cells, which once again, we'll get to talk more about that process later in our course when we're focusing on the adaptive immune system. But for now, this here concludes our brief lesson on the monocytes and how they can develop into macrophages and dendritic cells. We'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.