Immunity | Definition, Types & Importance
Table of Contents
- What is Immunity?
- Types of Immunity
- Natural Immunity Vs. Vaccinations
- The Importance of Immunity
- Lesson Summary
What are the types of immunity?
Innate immunity, our first line of defense, is our body's natural protection including skin and mucous membranes. Our humoral response, with inflammation and fever, is also part of our innate immunity. Adaptive (acquired) immunity occurs after infections agents have made it through and include B cells and T cells. Antibodies recognize specific pathogen antigens to disable and incapacitate them. These antibodies can be given across the placenta (passive immunity) or through active immunity after exposure occurs.
What is immunity and why is it important?
Immunity is the ability of our bodies to protect against invading pathogens such as bacteria or viruses. This can occur naturally through exposure or through vaccination. After an immune response has been activated the body is able to fight off future infections.
Table of Contents
- What is Immunity?
- Types of Immunity
- Natural Immunity Vs. Vaccinations
- The Importance of Immunity
- Lesson Summary
Immunity is the ability of our bodies to protect against invading pathogens such as bacteria or viruses. Microscopic invaders surround and outnumber us, and our immune system is responsible for making sure to keep out the dangerous variety or eradicate those that manage to enter our bodies. The first line of defense in our body includes the skin. The openings in our skin are further guarded by mucous, saliva, tears, and acidic environments.
If pathogens are able to pass through the first line of defense they meet phagocytes which ingest invading organisms. These "eating cells", part of our second line of defense, are considered to be a non-specific defense mechanism because they eat invaders without regard to what type of invaders are there. The image below shows an orange bacteria entering from the top. A phagocyte will leave the blood vessel, locate the invader, and ingest the invader indiscriminately.
Invaders that make it past the phagocytes will meet with further defenses - this time more specific. This segment of immunity depends on the recognition of antigens, name tags that invaders carry that identify them as non-self. The following are other types of immune system cells that an invader may come into contact with.
- Leukocytes, white blood cells, help the body remember and destroy invaders.
- Neutrophils fight bacteria.
- Lymphocytes (B lymphocytes) find target cells and alert the rest of the immune system. They can also make antibodies to lock onto antigens. These antibodies can stay in the body to fight future invaders.
- Lymphocytes (T lymphocytes) destroy invaders as well as infected cells.
The cell types above, in addition to the first two layers of defense, comprise the definition of immunity: the ability of a body to protect against invading pathogens.
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The two basic types of immunity are innate and adaptive. Innate immunity is our body's natural protection. Our dermis (skin) provides a barrier to keep invading pathogens out and other openings in our body are protected with mucus, acidic secretions, and physical reactions such as coughing or sneezing (to expel the invader). Innate responses can also include inflammation and fever caused by humoral immunity - a component of innate immunity.
If a pathogen passes this first barrier adaptive immunity takes over. Adaptive immunity, sometimes referred to as acquired immunity, occurs only after exposure to pathogens. This can be done through infection or a vaccine. While innate immunity is non-specific and defends against all invaders, adaptive immunity is a defense built against specific invaders.
Innate Immunity
Innate immunity is often referred to as the first line of defense. The term innate refers to what an individual is born with, so innate immunity is one's natural protection. Skin keeps pathogens from entering the body, mucous membranes in our nose and wax in ears keep bacteria and viruses literally stuck to them, stomach acid destroys invaders, and physical actions such as coughing and sneezing expel would-be invaders. Inflammation and fevers are also part of this early, innate immunity. Innate immunity is one of the basic types of immunity that our immune system gives us, attempting to keep all invaders out from the start.
Adaptive Immunity
Adaptive immunity is often called acquired immunity and occurs following exposure to a pathogen that has made it past the first and second line of defense, the skin, mucous, and humoral responses. Pathogens carry antigens, similar to molecular name tags, that our immune system can recognize and respond to. This exposure triggers the immune system to produce antibodies , Y-shaped molecules, that can attach to invading antigens. It is important to note that this third line of defense is specific - antibodies fit precisely to antigens and, therefore, the antibodies that recognize mumps will not recognize chickenpox. Adaptive immunity is one of the basic types of immunity that our immune system gives us. Adaptive immunity can be passive (during development) or active (after infection).
Passive Immunity
During passive immunity the body is given antibodies as opposed to creating its own. This occurs during development as the mother transfers antibodies to pathogens she is exposed to across the placenta. These antibodies can also be given through breast milk. While protection during passive immunity is immediate, it is short-lived and does not provide long-term immunity.
Active Immunity
Within this third line of defense in active immunity, there are two types of leukocytes (white blood cells). B cells recognize pathogenic invaders and T cells recognize when a cell is infected. B cells produce antibodies that bind to antigens and disable or destroy them. After the B cell has attached to its pathogen and initiates humoral immunity, it can also trigger more B cells to be created, referred to as memory cells, which can hasten the immune response if future infection from that particular pathogen occurs. Building a B cell army is not a quick process and often takes time, however, once the memory cells have been created long-lasting immunity, and the ability to fight future infection occurs. When the process occurs from exposure to infection it is referred to as natural immunity. This can also occur through artificially acquired active immunity by using vaccination where a weakened or killed form of the pathogen is given to the individual to stimulate B cell antibody and memory cell production. More recently vaccines can contain only parts of, or instructions for, the pathogen to stimulate an immune response.
The T cell response is different from B cells in that T cells attack infected cells. All cells contain their own antigen nametags distinguishing them from surrounding cells. Infected cells, however, also display molecules called MHC proteins (major histocompatibility complex proteins) that indicate the infection has taken place. These markers, displaying these familiar proteins, allow T cells to recognize and destroy them before the pathogen can spread to surrounding cells.
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Natural immunity occurs through exposure to a pathogen. When the B cells locate and attach to the invading antigen nametags on the invaders they can create an immune response of increased matching antibodies and increased B cell memory cells. This interaction can also cause T cells to recognize infected cells and make memory T cells. While protection would be long-lasting, the risk is that the individual must first survive the initial infection. The immune response is larger and faster upon future infections (that is what immunity entails) however, the first exposure can cause the individual to become very ill while the immune system fights.
Vaccinations attempt to circumvent the initial slow response to first-time infection by priming the body, preparing it to face the invaders that are prevalent in the area. Early vaccinations attempted to give small amounts of the pathogen to trigger a response. Unfortunately, this sometimes caused the initial disease since the pathogen was still entering. Later dead pathogens or pathogen parts were used which could cause the secondary immune response without infection. Unfortunately, these were difficult to grow in large quantities and had specific procedures necessary to ensure the pathogen was killed. Today vaccines can use DNA or RNA from the pathogen itself to create an artificial bit of molecule that resembles the pathogen, triggering a secondary immune response without any exposure. Vaccines can cause side effects and individuals should be sure to consult with a medical professional about the best course of action for their immune system and the local prevalent pathogens.
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We are surrounded by microscopic organisms, pathogens, bacteria, and viruses every day. Our immune system's job is to create a barrier to keep them out (innate) and then fight any that get through (active). Creating memory cells decreases the response time to ensure minimal infection and to address pathogens that are in the geographic region. Without an immune system our cells, and therefore our bodies, would quickly succumb to the millions of bacteria and viruses we come into contact with every day.
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Immunity is the ability of our bodies to protect against invading pathogens such as bacteria or viruses. From the initial first line of defense, our skin and mucous meant to keep pathogens out, to the phagocytes (cells eating invaders indiscriminately), to the specific lines of defense with B and T cells, our bodies work hard every day to keep us free from infection.
Innate immunity, our first line of defense, is our body's natural protection including skin and mucous membranes. Our humoral response, with inflammation and fever, is also part of our innate immunity. Adaptive (acquired) immunity occurs after infections agents have made it through and include B cells that recognize invader antigen nametags and T cells that recognize infected cells. Antibodies recognize specific pathogen (antigens) to disable and incapacitate them. These antibodies can be given across the placenta (passive immunity) during development and through breast milk or through active immunity after exposure occurs. B cells create antibodies as well as memory cells so that future infections are dealt with quickly. T cells recognize and destroy infected cells by recognizing familiar proteins (MHC proteins) and can also make memory cells to decrease secondary exposure-response time. As opposed to natural immunity where exposure occurs from everyday living, vaccination, is artificially acquired active immunity creating a secondary immune response, can create immunity without initial exposure. This can be done using a dead pathogen, part of a pathogen, or even (most recently) artificial parts of pathogens that white blood cells recognize and respond to. Immunity, whether, innate, adaptive, active, passive, or vaccine-induced allows us to combat the millions of pathogens that surround us every day.
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Video Transcript
What is Immunity?
If you could have a superpower, what would you choose? Super speed or strength? Invisibility? Have you ever considered immunity?
Immunity? How is that a superpower? Immunity means being protected from something and being unaffected or not bothered by it. Let's say you have immunity to heat - this means heat can't bother you whatsoever - walking up to hot molten lava (about 2,000 degrees F!) would be no different than walking up to a river.
Amazingly, we already have a form of this superpower because of our immune system. Our immune system protects us and helps fight off disease. Microorganisms, small microscopic organisms, and viruses are everywhere. Ever thought about how many are on that door you just opened? Many microbes and viruses can cause disease and are termed pathogens.
With all these pathogens around, why aren't we sick every single moment? The answer is our immune system gives us different types of immunity to protect us from disease. There are two main types of immunity: innate, also called natural or inherited, and adaptive.
Innate Immunity
Plants and animals have what is called innate immunity. Innate immunity is the first line of defense against pathogens. It involves several cell types, proteins, and even an organ. The organ involved is your skin. Yes, skin is part of the first line of defense. It protects you and prevents pathogens from getting inside your body.
So, what are some ways a pathogen gets inside? Air, food, or a break in the skin are some ways a pathogen enters. A pathogen entering through food or air has mucus to go through. The mucosal surfaces prevent pathogens from attaching to cells and causing disease. A set of proteins called the complement system is also involved. The complement system attacks the pathogen and marks it for destruction.
A pathogen getting through skin and mucus will have to deal with several types of cells including phagocytes, eating cells, and natural killer (NK) cells before it can cause disease. Pathogens have warning flags on their surface that say: 'I don't belong here'.
Neutrophils, macrophages, and dendritic cells are all phagocytes. They recognize the warning flag, attack the pathogen, and eat it - a process known as phagocytosis. If a pathogen is too big for one cell alone, several cells attack at once.
NK cells on the other hand, identify infected cells (host cells) and activate the host cell's death receptor pathway or give the cell a lethal injection (injecting enzymes that degrade proteins). Host cells even try to fight back by turning off machinery that would help the pathogen and sending out distress signals.
If pathogens make it through all this, it's time for adaptive immunity to step in, and they do this with the help of dendritic cells.
Adaptive Immunity
Adaptive immunity works slower than innate, and is more specific. There are two types: passive and active. Passive immunity occurs when antibodies are passed from one person to another, as through transfusion for example.
The active immunity involves two types of white blood cells - T-cells and B-cells. Dendritic cells, after they have eaten and digested the pathogen, present the pathogen pieces to T-cells, which activates (turns on) the T-cells.
Cell-Mediated Immunity
T-cells are formed in the thymus and cruise around until activated. Since T-cells require direct contact with other cells, T-cell immunity is termed cell-mediated immunity. Activated T-cells become helper cells (TH) and cytotoxic (killer) T-cells. They recognize and cause the destruction of infected cells.
But how do T-cells tell friend from foe? Cells have a marker that they show on their membranes. This marker is unique from person to person and one of the factors that must be considered in organ transplants. If the marker is missing, or doesn't display familiar proteins, the cell is chosen for destruction.
Humoral Immunity
T-Helper cells can activate B-cells. B-cells are formed in the bone marrow. They are responsible for antibody production. Antibodies (Ab) are Y-shaped proteins produced in response to foreign material. Antibodies recognize tags called antigens, anything that causes Ab production.
Each B-cell is pre-made to be specific to an antigen. When the antigen appears, the B-cell that has the antibody for it is activated and begins to make and secrete (send out) antibodies. B-cells even produce antibodies for antigens created by scientists, not normally found in nature! Antibodies attack only the antigen, so while it keeps pathogens from entering and stops more from forming, it can't destroy an infected cell - only T-cells and the innate immunity can.
Once activated, the B-cells either form plasma cells (Ab producing cells) or memory cells. These memory cells are more quickly activated so next time the antigen dares to show itself, it's dead meat.
Lesson Summary
Let's review! Immunity is a state of being protected from something and unaffected by it. The immune system gives us innate and adaptive immunity. Innate is the first line of defense and found in plants and animals. It involves neutrophils, macrophages, NK cells and dendritic cells. Adaptive immunity (active and passive) occurs slower and is more specific. Active adaptive immunity involves cell-mediated immunity, T-cells, and humoral immunity (B-cells). There are two types of T-cells - cytotoxic and helper T-cells - that identify and cause the destruction of infected cells. B-cells cannot destroy infected cells, but produce antibodies that target disease-causing antigens.
Types of Immunity: Main Points
Innate | Adaptive |
---|---|
*First line of defense *First layer is the skin, mucous, and the complement system *Includes cells that identify and eat pathogens |
*Includes cell-mediated immunity and humoral immunity *T-cells identify and destroy infected cells *B-cells produce antibodies that target disease-causing antigens |
Learning Outcomes
When you are done, you should be able to:
- Describe the body's layers of defense against pathogens
- Explain what the immune system is, its components, and how it works
- State the difference between innate and adaptive immunity
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