Innate Immunity - Online Tutor, Practice Problems & Exam Prep

The innate immune system serves as the body's first line of defense against pathogens, employing a nonspecific response. Key components include the skin, mucus, and leukocytes like neutrophils and macrophages, which utilize phagocytosis to eliminate invaders. Pattern recognition receptors, such as Toll-like receptors, identify pathogen-associated molecular patterns (PAMPs) to initiate immune responses. The inflammatory response, characterized by swelling and recruitment of immune cells, is crucial for managing infections. Additionally, the complement system aids in lysing pathogens, enhancing the overall immune defense.

Innate Immune Response

Hi. In this video, we'll be talking about the innate immune system, your body's first line of defense against pathogens. Now, the innate immune system is going to mount a non-specific response, as opposed to the specific response that the adaptive immune system will launch. The skin is basically the front lines of your body's defense systems. It's sort of like the moat to the fortress and it's going to protect the inside of your body from any pathogens that might want to enter. Of course, it's an imperfect barrier. Not only are there holes in it, like you can see this nostril right here, but it can also be penetrated in other ways, it can be broken open through physical injury, allowing pathogens in, so it's the front line but very much so not the final line of defense.

Now, all of the openings to your body will generally have some mucus surrounding them, its purpose is to trap foreign invaders. So, you know we have mucus in our nostrils because that's an opening to the outside, we could potentially breathe in pathogens like you see happening here in this image, so the goal is that if we do breathe in some bad stuff, it's just going to get stuck in our mucus and then we will expel it like you see this guy doing here. It's worth noting that mucus also can contain antimicrobial enzymes. So, not only will it trap those invaders, but it might actually kill them while they're trapped. Kind of like a roach motel. You know, the pathogens check-in but they don't check out.

Now, you will also find what are called lysozymes, which are enzymes that break down bacterial cell walls and you'll find these associated with tissues that, you know, can have pathogens entering from the outside. So it's just another sort of non-specific defense, in these vulnerable areas of the body.

Now, the innate immune response is going to be that first response to pathogens, it's going to be initiated by leukocytes which are those white blood cells and, again, it's going to be nonspecific as opposed to the specificity of the adaptive immune system. Now, since it's nonspecific, the immune cells have to be able to recognize the pathogens and they're going to do that through what is called pathogen-associated molecular patterns or PAMPs and basically, these are just molecules that are commonly displayed by pathogens and will not be displayed by human cells or, you know, cells of the organism's body so that it knows they're foreign. A perfect example of this is the lipopolysaccharides that you find on the surface of bacterial cells. You can see one right behind my head here and, basically, this is going to be just one of those pathogen-associated molecular patterns, not super specific, you know, like lots of different types of bacteria are going to have these but it's a way to tip our immune system off that, hey, there's some bacteria hanging out here.

So, how does our immune system recognize it? Well, our immune cells will have what are called pattern recognition receptors that are membrane receptors that identify those PAMPs. So these PRRs are going to identify the PAMPs and a specific type of pattern recognition receptor that you will see in a lot of immune cells are these Toll-like receptors as they're called, or TLRs and, again, those are a type of pattern recognition receptor that is found on some immune cells and what's special about them is they will initiate a signaling cascade, which will help recruit other immune cells and lead to a mounted immune response.

One specific type of Toll-like receptor focuses on those lipopolysaccharides from bacteria, those PAMPs that bacteria have. It will release cytokines in response to binding with the lipopolysaccharide and those cytokines are signaling molecules that will attract other immune cells and help mount the immune response and you can see those TLRs right here, this is our cell membrane, they are protruding out from the cell surface bind that lipopolysaccharide that is expressed by bacteria and that binding, as you can hopefully see, will initiate a whole cascade of responses within the cell that ultimately will lead to, for example, releasing those cytokines. With that, let's flip the page.

Innate Immune Cells

One of the main weapons that the cells of the innate immune system have against pathogens is phagocytosis. It allows them to remove these pathogens and prevent them, hopefully, from infecting any cells. Now, there are going to be 3 types of cells that will be performing phagocytosis in the innate immune system. Those are neutrophils which are phagocytes, or cells that perform phagocytosis, that will circulate in the blood and they're going to be attracted to infected tissues by cytokines, those messengers, or those messenger molecules, those chemical signals that will attract cells of the immune system. So, if I jump out of the way here, you can see a neutrophil circulating in the blood, you know, here we have a red blood cell. And it's going to circulate in the blood and look for these cytokines. And it's going to be attracted to them. Go in. Look for those pathogens that you can see here. So, that's a pathogen and it's going to phagocytose them. It's going to eat them up. And you can see what that phagocytosis kind of looks like here. The cell will actually pinch its membrane around the thing it's trying to engulf and basically suck it in in a vesicle. And here in this particular cell, it's actually going to digest what it takes in and break it down like you see there. Now, neutrophils are kind of like the mobile warriors. They're going to go seek and destroy these pathogens but macrophages are more like defenders. They tend to hang out in tissues and organs that will frequently encounter pathogens, and they'll mainly be found around the lymphatic system and especially lymph nodes because, you know, there are going to be a lot of pathogens that cycle through there. And, in addition to macrophages, you'll find dendritic cells in those areas too. And these dendritic cells are also phagocytes except their job is a little different. Gobble up the pathogen they find like, for example, the macrophages. Its job is just to gobble up any pathogen it finds; so like here's a bacteria it's going to gobble it up, remove it. Dendritic cells are there to actually take what they phagocytose and use it to stimulate the adaptive immune system. So, their job is super important because they're going to bridge the innate immune system and the adaptive immune system and they're going to help trigger an adaptive immune response. So, these phagocytes are not the only innate immune responses our body has, some other tactics they can use. You might not think of it because, you know, we tend not to deal with this in our day-to-day anymore but, you know, we used to have to worry about parasites much more often. We actually have these immune cells called eosinophils that are there to defend against multicellular parasites. And we also have some defenses against viruses. There are these cells called natural killer cells, cell death. And, if I jump out of the way here, you

Inflammatory Response

The inflammatory response is a special type of innate immune response that will result in swelling of a region. So, if you, for example, fall and scrape your knee, let's say. Following that injury, platelets are going to form clots to seal off that site and prevent blood loss and also hopefully prevent any nasty stuff from getting into the wound. Now, those wounded tissues and the macrophages around them are going to release chemokines which are basically like a special type of cytokine that recruits cells to the infection site. Now, mast cells, which are a special type of immune cell, they are a type of granulocyte, their main job, or one of their main jobs, is to release histamine and this is a chemical signal that is going to constrict blood vessels in the area and that is going to not only reduce blood flow to the area but also prevent blood loss from the area. And part of the mast cell's job is also to signal the peripheral blood vessels to the infection site. So, you know, the local blood vessels are going to constrict. The blood vessels in the area around that are actually going to dilate and that combination of dilation and the chemokines that are being secreted is a major signal to neutrophils to mobilize, get there right away, and go to that infection immunosuppression site and release cytokines to recruit more immune cells. So hopefully, you can see that immune responses are sort of like escalations, there is an initial signal of help, a bunch of immune cells move in and, you know, they kind of shout help and more guys come in, and the signals and the response grows and grows. It's an escalation to recruit more and more cells to that area to deal with any pathogens or infection and, here I am trying to sort of cheat and use the same figure but pretend that this time, these are histamines and also cytokines and stuff. I don't have a great image unfortunately to illustrate the inflammatory response but it is a very similar process and this is really a theme you'll see throughout the immune system is cells secreting a signal to recruit other cells to that area to mount some type of immune defense. So that's all I have for this video. I'll see you guys next time. Stay healthy.

Here’s what students ask on this topic:

What is the role of the skin in the innate immune system?

The skin acts as the first line of defense in the innate immune system, serving as a physical barrier to prevent pathogens from entering the body. It functions like a moat around a fortress, protecting internal tissues. However, the skin is not an impenetrable barrier; it has openings like nostrils and can be breached through injuries. Additionally, the skin is often associated with antimicrobial enzymes and lysozymes that break down bacterial cell walls, providing an extra layer of defense. Overall, while the skin is crucial in preventing pathogen entry, it is supported by other immune mechanisms for comprehensive protection.

How do leukocytes recognize pathogens in the innate immune system?

Leukocytes, or white blood cells, recognize pathogens in the innate immune system through Pathogen-Associated Molecular Patterns (PAMPs). PAMPs are molecules commonly found on pathogens but not on human cells. Leukocytes have Pattern Recognition Receptors (PRRs) on their membranes that identify these PAMPs. A specific type of PRR, known as Toll-like receptors (TLRs), can bind to PAMPs like lipopolysaccharides on bacterial surfaces. This binding initiates a signaling cascade that recruits other immune cells and mounts an immune response, often involving the release of cytokines to attract more leukocytes to the infection site.

What is the function of phagocytosis in the innate immune system?

Phagocytosis is a critical function in the innate immune system, allowing cells to engulf and digest pathogens. Three main types of phagocytic cells are involved: neutrophils, macrophages, and dendritic cells. Neutrophils circulate in the blood and are attracted to infection sites by cytokines. Macrophages reside in tissues and organs, especially around the lymphatic system, and act as defenders by engulfing pathogens. Dendritic cells also perform phagocytosis but primarily serve to present antigens to the adaptive immune system, bridging innate and adaptive immunity. This process helps eliminate pathogens and prevents infections from spreading.

What triggers the inflammatory response in the innate immune system?

The inflammatory response in the innate immune system is triggered by tissue injury or infection. When an injury occurs, platelets form clots to prevent blood loss and block pathogens. Wounded tissues and macrophages release chemokines, which recruit immune cells to the site. Mast cells release histamine, causing local blood vessels to constrict and peripheral blood vessels to dilate, increasing blood flow to the area. This combination of signals attracts neutrophils and other immune cells, which release cytokines to recruit even more cells. The result is swelling, redness, and heat, which help contain and eliminate the infection.

How does the complement system contribute to the innate immune response?

The complement system is a group of proteins found in the blood that enhances the innate immune response. These proteins are activated by pathogen-associated molecular patterns (PAMPs) or antibodies. Once activated, they form pores in the membranes of pathogens, causing the cells to lyse and die. This process, known as complement-mediated lysis, helps eliminate pathogens from the body. The complement system also aids in opsonization, marking pathogens for easier recognition and phagocytosis by immune cells, thereby amplifying the overall immune defense.