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.