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Question Video: Identifying Physical Barriers to Pathogens Biology

Which statement is an example of a physical barrier to pathogen entry? [A] Complement proteins in blood [B] Keratinized layers of epidermis [C] Hydrochloric acid in the stomach [D] Oils and salt in sweat [E] Lysozyme in tears and saliva


Video Transcript

Which statement is an example of a physical barrier to pathogen entry? Complement proteins in blood, keratinized layers of epidermis, hydrochloric acid in the stomach, oils and salt in sweat, or lysozyme in tears and saliva.

Before we answer this question, let’s first define what a pathogen is. A pathogen is a biological agent that can cause disease. A great example of a pathogen is a virus, some of which can enter our body and infect our cells through the respiratory system. And once a pathogen has entered the body, it can use our body’s resources to multiply and produce other pathogens. This is what can make us sick. Because there’s pathogens everywhere, our body has developed systems for keeping these pathogens out. We can call these defense barriers. These can be classified as chemical, where different chemicals or enzymes can be used to stop pathogens, or physical, where impenetrable structures are used to block entry.

Let’s look at these different answer choices in the question, starting with the chemical barriers. There are many different chemical barriers in our body. Let’s start with lysozyme, but first we’re going to need a pathogen to work with. This apple looks really tasty, except it’s covered with pathogens. But you’re hungry, so you eat it anyway. So, what stops these pathogens from entering the body and causing an illness? One of these chemical barriers occurs in the mouth when you start to chew on the apple. So, let’s zoom in to see what’s happening.

In the saliva is an enzyme called lysozyme that’s able to cleave certain components of the bacterial cell wall. This can cause the cell wall to rupture or lyse, and the bacterium dies. But what if the pathogen is able to bypass the effects of lysozyme? Not to worry! The stomach is another dangerous place for pathogens. The stomach contains hydrochloric acid. And when pathogens enter the stomach, this highly acidic environment is able to kill most pathogens. However, some pathogens may have adaptations that allow them to bypass this acidic environment. These can move on to other parts of the body. Ultimately, some pathogens can find themselves in the bloodstream, and even this can be a hostile environment.

In the blood, there are a group of proteins that are collectively known as complement. These proteins can attach themselves to the surface of some bacteria. This can form pores in the membrane, which can cause the bacterium to lyse and die. In addition, complement can help recruit cells of the immune system to help combat pathogens. If the complement system fails, the immune system has other methods in place. But we’ll save that discussion for another time.

So far, we’ve discussed what happens to pathogens when they get inside the body. But what about the outside? There’s chemical barriers in place there as well. For example, let’s look at the skin. Here’s a section of skin that shows a hair follicle, some blood vessels, and the sebaceous gland. There are gaps between the hair follicle and the skin that are tiny but large enough for pathogens to enter. The sebaceous gland produces a substance called sebum that is composed primarily of triglycerides and other chemicals that make it very oily, and this can form a protective seal in these gaps. These seals can prevent pathogens from entering. In addition, sweat contains salt, which can be harmful to bacteria.

Besides chemical barriers, our body is also equipped with physical barriers that can prevent pathogens from entering. A good example of this is with a specialized layer that’s present in the epidermis of the skin. The topmost layer of our skin is made up of very tough and impenetrable skin cells that get this property from a high amount of a protein called keratin. Pathogens might try to bypass this layer of skin. But because it’s so tough, they can’t penetrate this layer to enter the body, which is what makes this a good example of a physical barrier. Therefore, that keratinized layers of epidermis is an example of a physical barrier to pathogen entry.

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