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This lesson will cover the major anatomical barriers to infection of the innate immune system. We’ll explore skin flora, gut flora, gastric acid, lysozymes, and the mucociliary apparatus.

Anatomical Barriers of the Immune System

Your immune system is comprised of two main halves: the innate immune system and the adaptive immune system.

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The former is the primary line of defense in your body’s fight against pathogens, or invaders, that are always trying to harm it. This very first line of defense is made up of anatomical barriers, and their components, that try to stop an invader from getting into your body in the first place. This is what we’ll focus in on in this lesson.

The Skin’s Role in the Immune System

When a pathogen tries to enter your body it must choose a path to get in. These paths can be through a crack in the skin, through the nose, or through the digestive system, among others. One of the most visible barriers to infection your body has is the skin. If your blood vessels were like a tunnel running underneath your skin, then your skin would be a physical roadblock of sorts on the highway leading to that tunnel.

If a pathogen, like bacteria, were to try to ram through the roadblock like a car, it would most likely be very unsuccessful. The skin is pretty strong in this respect, kind of like a giant concrete barrier. However, if the roadblock, your skin is damaged due to something like a cut, the car would be able to pass right through the shattered roadblock and into the tunnel. Once inside the tunnel, it can spread all over the body to cause you a lot of damage.In addition to its purely anatomical way of defending against pathogens, your skin has something known as skin flora, which is bacteria on the human skin.

Some of these bacteria are commensal, or basically harmless, while others are mutualistic and actually help to fight off pathogenic skin flora. You can imagine these friendly bacteria as friendly cars on the road that set themselves up across the highway, just like the roadblock, as an added layer of protection against a pathogen trying to get into the tunnel.

The Gastrointestinal Tract

Besides entering through the skin, another way by which a pathogen can try to gain access to your body is through the gastrointestinal tract, which includes the esophagus, stomach, and intestines. However, this is a very treacherous route through very dark and dangerous quarters. It’s like spelunking, or going into caves. Once a pathogen enters the GI tract, it sees that this GI tract is very dark and very perilous. In this system of caves and tunnels that connect those caves, our spelunker can fall into a highly acidic digestive fluid called gastric acid that will kill it very quickly.

If falling into an acidic lake in a cave wasn’t bad enough and the invader manages to get through, the intestines have a very rude awakening for the pathogen. The intestines are full of gut flora, or bacteria in the digestive tract, some of which are mutualistic bacteria involved in digestion and the defense of the human body. This gut flora is basically filled with dangerous bats ready to fight for their home, the intestinal tract, and kill off any invader trying to harm it. They’re like the mutualistic skin flora, but in your gut.

As a cool side note, there are about 100 trillion bacteria in your digestive tract. This is 10 times more than the amount of cells that make up your entire body. In that respect, you’re basically a giant walking vat of bacteria more than you are a human being.

The Eyes and Respiratory Tract

Regardless, pathogens don’t just take to the roads, tunnels, and caves. They love air travel as much as most of us.

When they take to the air, they like to attack us through our respiratory tract. Meaning, they’ll be inhaled via the mouth or nose and go into the back of our throat, perhaps down the trachea, and into our lungs. Thankfully, as this little airplane, the pathogen, enters the respiratory tract it encounters a wide variety of defense systems part of the innate immune system.

These include mucus, saliva, and lysozymes, which are enzymes found in tears, mucus, and saliva that help to kill bacteria.As I’m sure you know, mucus and saliva are pretty wet, and act like high humidity would in the air on the pathogen airplane. This high humidity would slow down or put a stop to our pathogen completely. However, if the pathogen has engines that are powerful enough to get past the humidity, the saliva in the mouth and mucus in the nose, there are other defense mechanisms ready to spring into action.For example, they may encounter something called the mucociliary apparatus, which will help to clear away any pathogen that crashes or gets stuck in the mucus.

This is a system composed of mucus covered cilia that clears the respiratory tract of foreign material. You can liken the cilia, which are like little hairs, to airport crews. If a pathogen, our airplane, makes a crash landing on the runway in the mucus, the cilia will spring into action to clean up the mess, and will move it from the lower reaches of the respiratory system, into the upper reaches, where it can be either sneezed or coughed out or swallowed into the deep, dark, caves of the gastrointestinal tract.Keep in mind that we covered the main methods of the anatomical components of the innate immune system. There are plenty of other ways by which your body protects itself using physical or chemical means. For example, the secretion of things like urine, tears, or substances found in tears and urine can serve to remove pathogens from your system or kill them outright.

Lesson Summary

As a review, let’s go over the anatomical portions of the innate immune system that pathogens can use in order to enter your body, and the different methods these body systems use in order to protect you.Your skin can protect you physically. Like a band-aid over a wound, your skin is like a band-aid over your entire body. In addition to its purely anatomical way of defending against pathogens, your skin has something known as skin flora, which is bacteria on the human skin, some of which is mutualistic and helps to fight off pathogenic bacteria on the skin.

Pathogens can use your GI tract to invade your body as well. However, the stomach has a highly acidic digestive fluid called gastric acid that will kill many invaders very quickly and the intestines are full of gut flora, or bacteria in the digestive tract, some of which are mutualistic bacteria involved in digestion and the defense of the human body.If the invaders try to avoid the skin and GI tract and try to make their way in via the respiratory system, then mucus, saliva, and lysozymes, which are enzymes found in tears, mucus, and saliva that help to kill bacteria, are used to help protect you. In addition, the respiratory system has a mucociliary apparatus, which is a system composed of mucus-covered cilia that clears the respiratory tract of foreign material.

Learning Outcomes

After this lesson, you should recognize that the body’s immune system is constantly being bombarded by infection through the skin, GI tract and respiratory system. You may realize, however, that the immune system is constantly vigilant and works overtime to clear the body of the majority of disease through lysozymes, flora, gastric acid, and cilia.

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