Understand what innate immunity is, and learn about inflammation, neutrophils, macrophages, complement proteins, natural killer cells, and eosinophils.
Updated: 01/20/2022
Innate Immunity Overview & Mechanisms
Innate Immunity
The human body is in a constant fight with the ubiquitous enemies, infectious microbes and pathogenic organisms. Some of these may remain extracellular, extracting nutrients from living tissues, while others may invade and live within animal/human cells, replicating and surviving well. These invaders range from being helpful (e.g., E. coli in the intestines) to being major pathogens which can be fatal (e.g., HIV). In spite of all these threatening situations, the human body manages to remain disease free most of the time. This is because it is equipped with a defense system against these deadly organisms. The system by virtue of which the body recognizes and defends itself against foreign and harmful substances or organisms is called the immune system. The immune system recognizes and responds by trying to destroy pathogens through several mechanisms. The two broad categories of the immune system are: 1) the innate immune system and 2) the adaptive immune system.
The innate immune system or innate immunity is the first line of defense and acts immediately and rapidly upon exposure to the foreign bodies or antigens (antigens are anything that elicit immune response). The innate immune system is composed of defenses or barriers that can be activated immediately to limit the spread and pathogenicity of pathogens, thereby protecting the body against them. The innate immune system is structured with multi-layered barriers and defense mechanisms. The types and mechanisms of the innate immune system are tabulated below with examples:
| Innate Immunity | Examples |
|---|---|
| Physical/Anatomical Barriers | |
| Skin, Mucous membrane, cilia, eyelashes, body hair | Skin acts as a physical barrier and prevents the entry of microbes. Cilia present in different structures of the respiratory system and beat in an upward direction to repel microorganisms that enter during breathing. |
| Secretions | |
| Mucus, bile, gastric juice, tears, saliva, sweat | Mucus coats the cell surfaces, making it difficult for microbes to bind to these cells. Lysozyme, secreted in tears and by sweat glands, breaks the cell wall of bacteria. |
| Physiological Barriers | |
| Body temperature, fever, acidic conditions | Body temperature and fever retard the growth of many microbes. The acidic environment of sweat inhibits microbial growth, and low pH gastric juices kill many ingested pathogens. |
| Innate Immune Cell Mediated Responses | |
| Neutrophils, macrophages, natural killer cells, eosinophils | Neutrophils and macrophages are phagocytes that ingest and kill the microbe. Natural killer cells mainly kill infected self-cells and some tumor cells by releasing granule contents onto them. |
| Inflammatory response | Inflammation is the process that deals with tissue damage and infection. It induces a leakage of vascular fluid and an influx of phagocytic cells at the site of infection. |
| Complement system response | The complement system involves a cascade of molecular events leading to enhanced phagocytosis and the killing of microbes. |
Innate immunity is the natural or intrinsic defense system with which one is born. It is inherited and present since the time of the birth of an individual and does not undergo much changes throughout their life. Innate immunity does not have any memory. That means the immune system does not remember encounters with previous infections and there are not many changes in its reactivity upon subsequent infections. Also, innate immunity poses a non-specific response as it does not differentiate between antigens and responds similarly against all.
Inflammation
Inflammation is the body's response and process to fight against pathogens, toxins, and injuries. It is a process of healing wherein immune cells are sent to the site of injury in response to certain chemicals (inflammatory mediators) from tissues, microbes, or from other cells, including mast cells and macrophages. Mast cells are central to the inflammatory process and are considered important inflammatory cells, as they release histamine and other inflammatory mediators on stimulation. This triggers vascular changes such as blood vessel dilation (vasodilation) and increased permeability of the blood vessel. Endothelial cells lining the blood cells start presenting adhesion molecules on their cell surface to which immune cells bind and stick to the wall of blood vessels. Due to increased permeability of the blood vessels, these immune cells squeeze out between the endothelial cells to the injured site. Increased blood vessel permeability also causes the leakage of serum proteins and fluids with anti-microbial properties into the injured site, causing edema (fluid accumulation), which results in swelling. Vasodilation causes increased blood flow that results in redness and heat. Pain receptors give rise to pain at the site of tissue damage. Thus, histamine released by mast cells acts as a chemoattractant to attract immune cells to the damaged site and initiate the inflammatory process. There are five key signs of inflammation, namely:
Redness, Heat, Swelling, Pain, and Loss of Function
Neutrophils
Neutrophils are phagocytes (phagocytes are specialized cells that engulf other cells, microbes or cell debris), typically the first cells to arrive at the site of an infection, as numerous neutrophils keep circulating in the blood stream at any given time. Neutrophils are also called polymorpho-nuclear cells (PMNs) because of the multi-lobed nature of their nuclei, which are produced in the bone marrow. Neutrophils possess granules which are capable of killing microbes in their cytoplasm, and hence are known as granulocytes. These short-lived neutrophils comprise the majority of white blood cells. Neutrophils have several different types of surface receptors that recognize specific molecules or patterns present exclusively on microbes. Some surface receptors are chemoattractant receptors, for attraction of neutrophils towards the microbes. Thus, the main function of neutrophils is to patrol the body, looking for the invading microbes and to immediately respond to inflammation to phagocytose them.
 |
Phagocytosis is the major mechanism by which microbes are removed from the body. It is a multistep process consisting of the following steps:
1. Movement of phagocytes towards pathogen
2. Attachment of pathogen to the phagocytes via surface receptors
3. Engulfing of pathogen through extension of cytoplasm and invagination of plasma membrane of neutrophils around it, resulting in the formation of phagosome
4. Fusion of lysosome with phagosome and formation of phagolysosome
5. Killing of pathogen and exocytosis (throwing out the digested cell material to the cell exterior)
Neutrophils are an important component of the innate immune system as they provide front line defense by responding first to the infection site and killing the microbes. The normal range of neutrophils is around 40-60% of the total white blood cell count. A higher level of neutrophils in the blood indicates an active microbial infection.
Macrophages
Macrophages are tissue-bound phagocytes, unlike neutrophils, which reside in the bloodstream. Macrophages are the part of the mononuclear phagocyte system derived from monocytes. The major function of macrophages is to phagocytize leftover microbes, cellular debris, and dead cells in the tissue; they're equivalent to a clean-up crew. Macrophages take different names while doing the same functions in different tissues; some of those names are:
- Kupffer cells in the liver
- Mesangial cells in the kidney
- Microglial cells in the brain
- Osteoclasts in the bone
- Alveolar macrophages in the lungs
 |
Neutrophils and macrophages are called professional phagocytes, as these cells are not only capable of containing and killing the microbes by phagocytosis, but they also process them for antigen presentation, and they play an essential role in initiating the adaptive immune response. Although neutrophils and macrophages have similar functions, they differ in the following ways:
The Innate Immune System
When people think of the immune system, the first thing that comes to mind is often antibodies or vaccines or some other aspect of acquired immunity to an infectious agent that the body has already seen. But how does the body protect itself against potentially dangerous bacteria and viruses that it has never seen before? The immune system needs a way to quickly respond to a variety of unknown invaders, whether they've been encountered before or not, and all of the aspects of the immune system that respond to these unknown threats contribute to our bodies' innate immunity or the ability of the immune system to respond to unknown threats.
 |
The innate immune system is capable of mounting a defense against a variety of different threats, including malignant or cancerous cells, viruses that have already infected a cell, multicellular parasites and any other potential pathogens or foreign invaders that are capable of causing disease that might enter the body through an open wound or body orifice. So, let's take a look at how the innate immune system works.
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| Neutrophil | Macrophage |
|---|---|
| Has multi-lobed nucleus | Has large, round-shaped nucleus |
| Is a granulocyte | Is an agranulocyte |
| Matures in bone marrow | Matures in tissue |
| Short-lived in the range of a few days | Long-lived in the range of weeks or months |
| Found in abundance in circulation | A lot less in circulation |
Complement proteins
Unlike the immune cells discussed above, the complement system consists of several interdependent serum proteins, which on sequential activation mediate protection against certain microbial infections. Complement proteins are glycoproteins designated as C1, C2, and so on. These proteins are mostly synthesized by hepatocytes and monocytes and keep circulating in the bloodstream. Once triggered, a series of molecular events begins that involves cleavage and activation of subsequent complement components into active form. The series of events ultimately leads to the formation of a membrane attack complex (MAC) that creates holes in the membrane of microbes and kills them. The complement system is non-specific and capable of targeting the body's own cells as well. Our own cells have complement disrupting membrane proteins which can disrupt the formation of membrane attack complexes by the complement system. Microbes lack such membrane proteins and are lysed by the complement system.
The major functions of the complement system are:
Initiation of inflammation by direct activation of mast cells
Chemotaxis (attraction of neutrophils to the infection site)
Opsonization (Increased attachment of the microbe to the phagocyte)
Cell lysis (formation of membrane attack complex and killing of microbes)
Natural Killer Cells
Natural Killer cells (NK cells) are immune cells which do not kill pathogens directly; rather, in order to stop the spread of an infection, NK cells destroy infected host cells. NK cells mark these cells by the absence of MHC-1 molecules on their cell surface. Upon the infection of cells by some viruses, the expression of MHC molecules reduces. This results in a decreased expression of MHC-I molecules in their cell surfaces than normal uninfected cells. This is an important mechanism, allowing NK cells to identify and ignore normal cells while killing infected or malignant cells. There are three types of cells that do not always express MHC 1 molecules on their surface:
- Red Blood cells
- Virus-infected cells
- Malignant cells
NK cells' mediated killing involves the release of granule contents onto the surface of the infected cell, inducing their apoptosis. The killing mechanism of NK cells is similar to that of cytotoxic T cells, except for the fact that NK cells can spontaneously kill target cells (virus-infected cells, cancerous cells) without prior activation, while cytotoxic T cells require priming by antigen-presenting cells. Moreover, a cytotoxic T cell is a part of the adaptive immune system so that it recognizes specific antigens and can maintain memory. Natural killer cells, being part of the innate immune system, respond rapidly as no prior activation of an antigen is required. The cytotoxic T cells are MHC-restricted and take days to respond while NK cells are MHC non-restricted.
Eosinophils
Eosinophils are a type of granulocyte which are primarily responsible for killing multicellular parasites. The granules of eosinophils contain free radicals and highly toxic proteins that can kill parasites. Eosinophils also contain an anti-inflammatory substance, histaminase, that dampens the effects of histamine released by mast cells during inflammation. They are present at low levels in the circulation and constitute approximately 1-6% of total white blood cells. Conditions like parasitic infection, allergies, or cancer cause the eosinophils' level to go up.
Lesson Summary
The innate immune system is the first line of defense of the body and it works rapidly and in a non-specific way, such that it does not differentiate among the pathogens and responds against them similarly. Innate immunity does not have any memory of the previous pathogen encounters, and thus, every time its reactivity and response towards them is fresh and the same. The key components of the innate immune system include physical barriers, physiological barriers, immune cells' mediated response, inflammatory response, and complement proteins' response. Inflammation is the body's response and process to fight against pathogens, toxins and injuries. Inflammation has the five following key signs:
- Pain: Due to pain receptors.
- Redness: Due to vasodilation and an increase in the blood supply to the blood vessels in the infected area.
- Loss of function: Due to tissue damage.
- Swelling: Due to increased permeability of blood vessels and building up of fluid.
- Heat: Due to vasodilation and increased blood flow, the affected area becomes warm.
The cells of the innate immune system act rapidly in killing the microbes using various mechanisms:
- Neutrophils- Phagocytize microbes and arrive first at the inflammation site.
- Macrophage- Tissue-based phagocytes; gets rid of leftover debris, bacteria, and dead cells
- Natural killer cells- Kills virus-infected cells and cancerous cells which do not express MHC-I molecules on their cell surface by releasing granules onto them.
- Eosinophils- Immune cells primarily responsible for killing multicellular parasites through its destructive granules containing highly toxic proteins and free radicals.
The complement system is the collection of interdependent serum proteins which undergo a variety of activation reactions, ultimately leading to the formation of a membrane attack complex (MAC) which kills the microbes by creating holes in their membrane. The self-cells of the body escape the complement system by the virtue of certain membrane proteins which disrupt the MAC.
Inflammation
Let's say that our friend Phil is rock climbing when he cuts himself on a sharp edge. It's not a bad cut, so Phil ignores it, but his immune system certainly doesn't! As soon as Phil sustains even the smallest injury, the potential for infection by a foreign invader dramatically increases. In fact, even with the most minor cuts, thousands, if not millions, of foreign entities, most of them bacteria, will enter the wound. So, whenever there is an injury, it is in the body's best interest to mount an immediate immune response to destroy all foreign invaders before they can establish a foothold.
The first response of the body to a localized injury or infection is inflammation or the immediate response of the body to injury or infection that includes an influx of blood plasma and immune cells to a localized area. Injured tissues release a number of signaling molecules, which cause blood vessel dilation, increase blood vessel permeability, and causes endothelial cells that line the blood vessels to present adhesion molecules on their surface. These adhesion molecules are recognized by immune cells in the blood or white blood cells, which bind to the adhesion molecules and stick to the blood vessel wall. The white blood cells then squeeze themselves between endothelial cells of the now more permeable blood vessels and enter the tissue surrounding the vessel.
The signaling molecules are immune cell chemoattractants that cause the immune cells to exit the blood vessel and migrate to the site of injury. One of the best known inflammatory signaling molecules is histamine, which is capable of dilating blood vessels, increasing blood vessel permeability, and causing immune cells to exit the bloodstream and enter the damaged tissue.
Cardinal Signs:
There are five signs that describe an inflammatory response.
 |
Neutrophils
The first immune cells that arrive at an injured site are mostly neutrophils, which are quick-response immune cells that recognize and destroy bacteria. Neutrophils are the most numerous type of white blood cell in the blood, and their job is to immediately respond to inflammation and kill bacteria by phagocytosis, which is the process by which a cell engulfs, destroys and digests another cell, bacteria or cellular debris.
 |
Neutrophils have several different types of receptors that recognize several different pathogen-associated molecular patterns that are specific to bacteria and not found on human cells. When the receptors bind to the bacteria-specific patterns, the neutrophil captures the bacteria and engulfs it, surrounding it with its plasma membrane. This intracellular compartment is called a phagosome because it is created during the process of phagocytosis. Lysosomes then fuse with the phagosome to add their digestive enzymes, which kill the bacteria and break it down into its component amino acids, nucleic acids and monosaccharides. These nutrients can then be used by the neutrophil or released into the tissue for use by other cells.
Macrophages
Macrophages are another type of immune cell that phagocytizes foreign organisms, dead cells and cellular debris. Macrophages are a little bigger than neutrophils. They live longer than neutrophils and stay in the tissue even after all of the pathogens are gone. They act like a clean-up crew that phagocytizes left over debris, bacteria and dead cells.
Complement
So far, most of the components of the innate immune system that we've talked about have been different types of cells, but there are non-cellular components of the innate immune system, too. Complement is a group of serum proteins which mark and destroy foreign microorganisms. Complement proteins are capable of forming a membrane attack complex on plasma membrane surfaces through a series of interactions between the different complement proteins. If the complement proteins are allowed to complete the membrane attack complex, it creates holes in the plasma membrane of its target and destroys it.
 |
Complement proteins are very non-specific; they will attempt to form membrane attack complexes on any plasma membrane they can. Now this might cause a problem if complement proteins formed membrane attack complexes on the body's own cells and destroyed them, but our cells don't let this happen.
Complement protein complexes that are forming on cell surfaces are fairly unstable and can be disrupted at various stages of completion by at least eight known membrane proteins that are expressed on the surface of all of our cells. So, while complement proteins try to form membrane attack complexes on any membrane surface, the body's own cells easily clear the proteins from themselves, but most bacteria and parasites don't have these complement disrupting proteins, so the membrane attack complex is formed, and the cell is lysed. However, certain types of microorganisms are resistant to lysis. In these cases, the complement proteins remain on the membrane and mark the microorganism as foreign. Neutrophils and macrophages have receptors to complement proteins and will phagocytize the marked invaders.
So, as you can see, complement serves to kill or mark all foreign cells and doesn't rely on recognizing specific organisms or molecules.
Natural Killer Cells
Natural killer cells are a type of immune cell that kills cells not expressing MHC 1 molecules on their surface. Like security guards in a restricted area, natural killer cells check the IDs of cells they come into contact with. For natural killer cells, or NK cells for short, the ID that they need to see is the MHC 1 molecule that normal cells express at all times on their cell surface. But there are three types of cells that do not always express MHC 1 molecules on their surface:
1. Red blood cells, which NK cells ignore for reasons that we won't go into here.
2. Cells infected with a virus
3.Malignant or cancerous cells
But the body doesn't have a quarantine room or a jail where it can put dangerous cells. Instead, the immune system can't take any chances, and the NK cells kill suspicious cells that do not express the correct MHC 1 molecule on their surface.
Eosinophils
Eosinophils are immune cells that attack multicellular parasites. These cells target multicellular parasites like worms by moving into position right up against the parasite and then releasing a host of destructive enzymes from their internal granules directly onto their target. Eosinophils also serve to ramp down the inflammatory response by arriving in the late stages of the response and releasing inhibitors of inflammatory molecules.
Lesson Summary
Let's review. Innate immunity is the ability of the immune system to respond to unknown threats. The innate immune system is capable of mounting a defense against a variety of different threats including any potential pathogens or foreign invaders that are capable of causing disease.
Inflammation is the immediate response of the body to injury or infection that includes an influx of blood plasma and immune cells to a localized area. The first immune cells that respond to inflammation are mostly neutrophils, which are quick-response immune cells that recognize and destroy bacteria. Neutrophils immediately respond to inflammation and kill bacteria by phagocytosis, which is the process by which a cell engulfs, destroys and digests another cell, bacteria or cellular debris. Macrophages are another type of immune cell that phagocytizes foreign organisms, dead cells and cellular debris. Macrophages stay in the tissue even after all of the pathogens are gone and act as a clean-up crew that phagocytizes all left over debris.
Complement is a group of serum proteins that mark and destroy foreign microorganisms. Complement proteins are capable of forming a membrane attack complex on plasma membrane surfaces through a series of interactions between the different complement proteins. If the complement proteins are allowed to complete the membrane attack complex, it creates holes in the plasma membrane of its target and destroys it. The body's own cells have several different membrane proteins that disrupt complement complexes and don't allow the complement proteins to form membrane attack complexes on the body's own cells.
Natural killer cells are a type of immune cell that kills cells that do not express MHC 1 molecules on their surface. Like security guards in a restricted area, natural killer cells check the IDs of cells it comes into contact with. For NK cells, the ID that they need to see is the MHC 1 molecule, which normal cells express at all times on their cell surface. NK cells kill suspicious cells that do not express the correct MHC 1 molecule on their surface, which is usually a sign that the cell is cancerous or infected with a virus.
Eosinophils are immune cells that attack multicellular parasites by releasing a host of destructive enzymes from their internal granules directly onto their target. Eosinophils also serve to ramp down the inflammatory response.
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Video Transcript
The Innate Immune System
When people think of the immune system, the first thing that comes to mind is often antibodies or vaccines or some other aspect of acquired immunity to an infectious agent that the body has already seen. But how does the body protect itself against potentially dangerous bacteria and viruses that it has never seen before? The immune system needs a way to quickly respond to a variety of unknown invaders, whether they've been encountered before or not, and all of the aspects of the immune system that respond to these unknown threats contribute to our bodies' innate immunity or the ability of the immune system to respond to unknown threats.
|
The innate immune system is capable of mounting a defense against a variety of different threats, including malignant or cancerous cells, viruses that have already infected a cell, multicellular parasites and any other potential pathogens or foreign invaders that are capable of causing disease that might enter the body through an open wound or body orifice. So, let's take a look at how the innate immune system works.
Inflammation
Let's say that our friend Phil is rock climbing when he cuts himself on a sharp edge. It's not a bad cut, so Phil ignores it, but his immune system certainly doesn't! As soon as Phil sustains even the smallest injury, the potential for infection by a foreign invader dramatically increases. In fact, even with the most minor cuts, thousands, if not millions, of foreign entities, most of them bacteria, will enter the wound. So, whenever there is an injury, it is in the body's best interest to mount an immediate immune response to destroy all foreign invaders before they can establish a foothold.
The first response of the body to a localized injury or infection is inflammation or the immediate response of the body to injury or infection that includes an influx of blood plasma and immune cells to a localized area. Injured tissues release a number of signaling molecules, which cause blood vessel dilation, increase blood vessel permeability, and causes endothelial cells that line the blood vessels to present adhesion molecules on their surface. These adhesion molecules are recognized by immune cells in the blood or white blood cells, which bind to the adhesion molecules and stick to the blood vessel wall. The white blood cells then squeeze themselves between endothelial cells of the now more permeable blood vessels and enter the tissue surrounding the vessel.
The signaling molecules are immune cell chemoattractants that cause the immune cells to exit the blood vessel and migrate to the site of injury. One of the best known inflammatory signaling molecules is histamine, which is capable of dilating blood vessels, increasing blood vessel permeability, and causing immune cells to exit the bloodstream and enter the damaged tissue.
Cardinal Signs:
There are five signs that describe an inflammatory response.
|
Neutrophils
The first immune cells that arrive at an injured site are mostly neutrophils, which are quick-response immune cells that recognize and destroy bacteria. Neutrophils are the most numerous type of white blood cell in the blood, and their job is to immediately respond to inflammation and kill bacteria by phagocytosis, which is the process by which a cell engulfs, destroys and digests another cell, bacteria or cellular debris.
|
Neutrophils have several different types of receptors that recognize several different pathogen-associated molecular patterns that are specific to bacteria and not found on human cells. When the receptors bind to the bacteria-specific patterns, the neutrophil captures the bacteria and engulfs it, surrounding it with its plasma membrane. This intracellular compartment is called a phagosome because it is created during the process of phagocytosis. Lysosomes then fuse with the phagosome to add their digestive enzymes, which kill the bacteria and break it down into its component amino acids, nucleic acids and monosaccharides. These nutrients can then be used by the neutrophil or released into the tissue for use by other cells.
Macrophages
Macrophages are another type of immune cell that phagocytizes foreign organisms, dead cells and cellular debris. Macrophages are a little bigger than neutrophils. They live longer than neutrophils and stay in the tissue even after all of the pathogens are gone. They act like a clean-up crew that phagocytizes left over debris, bacteria and dead cells.
Complement
So far, most of the components of the innate immune system that we've talked about have been different types of cells, but there are non-cellular components of the innate immune system, too. Complement is a group of serum proteins which mark and destroy foreign microorganisms. Complement proteins are capable of forming a membrane attack complex on plasma membrane surfaces through a series of interactions between the different complement proteins. If the complement proteins are allowed to complete the membrane attack complex, it creates holes in the plasma membrane of its target and destroys it.
|
Complement proteins are very non-specific; they will attempt to form membrane attack complexes on any plasma membrane they can. Now this might cause a problem if complement proteins formed membrane attack complexes on the body's own cells and destroyed them, but our cells don't let this happen.
Complement protein complexes that are forming on cell surfaces are fairly unstable and can be disrupted at various stages of completion by at least eight known membrane proteins that are expressed on the surface of all of our cells. So, while complement proteins try to form membrane attack complexes on any membrane surface, the body's own cells easily clear the proteins from themselves, but most bacteria and parasites don't have these complement disrupting proteins, so the membrane attack complex is formed, and the cell is lysed. However, certain types of microorganisms are resistant to lysis. In these cases, the complement proteins remain on the membrane and mark the microorganism as foreign. Neutrophils and macrophages have receptors to complement proteins and will phagocytize the marked invaders.
So, as you can see, complement serves to kill or mark all foreign cells and doesn't rely on recognizing specific organisms or molecules.
Natural Killer Cells
Natural killer cells are a type of immune cell that kills cells not expressing MHC 1 molecules on their surface. Like security guards in a restricted area, natural killer cells check the IDs of cells they come into contact with. For natural killer cells, or NK cells for short, the ID that they need to see is the MHC 1 molecule that normal cells express at all times on their cell surface. But there are three types of cells that do not always express MHC 1 molecules on their surface:
1. Red blood cells, which NK cells ignore for reasons that we won't go into here.
2. Cells infected with a virus
3.Malignant or cancerous cells
But the body doesn't have a quarantine room or a jail where it can put dangerous cells. Instead, the immune system can't take any chances, and the NK cells kill suspicious cells that do not express the correct MHC 1 molecule on their surface.
Eosinophils
Eosinophils are immune cells that attack multicellular parasites. These cells target multicellular parasites like worms by moving into position right up against the parasite and then releasing a host of destructive enzymes from their internal granules directly onto their target. Eosinophils also serve to ramp down the inflammatory response by arriving in the late stages of the response and releasing inhibitors of inflammatory molecules.
Lesson Summary
Let's review. Innate immunity is the ability of the immune system to respond to unknown threats. The innate immune system is capable of mounting a defense against a variety of different threats including any potential pathogens or foreign invaders that are capable of causing disease.
Inflammation is the immediate response of the body to injury or infection that includes an influx of blood plasma and immune cells to a localized area. The first immune cells that respond to inflammation are mostly neutrophils, which are quick-response immune cells that recognize and destroy bacteria. Neutrophils immediately respond to inflammation and kill bacteria by phagocytosis, which is the process by which a cell engulfs, destroys and digests another cell, bacteria or cellular debris. Macrophages are another type of immune cell that phagocytizes foreign organisms, dead cells and cellular debris. Macrophages stay in the tissue even after all of the pathogens are gone and act as a clean-up crew that phagocytizes all left over debris.
Complement is a group of serum proteins that mark and destroy foreign microorganisms. Complement proteins are capable of forming a membrane attack complex on plasma membrane surfaces through a series of interactions between the different complement proteins. If the complement proteins are allowed to complete the membrane attack complex, it creates holes in the plasma membrane of its target and destroys it. The body's own cells have several different membrane proteins that disrupt complement complexes and don't allow the complement proteins to form membrane attack complexes on the body's own cells.
Natural killer cells are a type of immune cell that kills cells that do not express MHC 1 molecules on their surface. Like security guards in a restricted area, natural killer cells check the IDs of cells it comes into contact with. For NK cells, the ID that they need to see is the MHC 1 molecule, which normal cells express at all times on their cell surface. NK cells kill suspicious cells that do not express the correct MHC 1 molecule on their surface, which is usually a sign that the cell is cancerous or infected with a virus.
Eosinophils are immune cells that attack multicellular parasites by releasing a host of destructive enzymes from their internal granules directly onto their target. Eosinophils also serve to ramp down the inflammatory response.
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Frequently Asked Questions
Are neutrophils part of the innate immune system or the adaptive immune system?
Neutrophils are a part of the innate immune system as they are the first cells to arrive and respond (phagocytize) at the infection site.
What is the difference between neutrophils and macrophages?
Neutrophils and macrophages both are immune cells and kill microbes by the phagocytosis process. Neutrophils are short-lived and found in the blood stream whereas macrophages are long-lived and tissue-bound phagocytes.
What type of cells cause inflammation?
Mast cells are central to the process of inflammation as they release histamines and other inflammatory mediators. These inflammatory mediators trigger vascular changes and thus initiate the inflammatory process.
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