In this video, we're going to talk about how the human microbiome can actually promote immune tolerance. Now before we continue, it's important to recall from some of our previous lesson videos that we've already discussed immune tolerance. And so, if you don't remember much about immune tolerance, be sure to go back and check out those videos before you continue here. Now that being said, it's important to note that both T and B cells must be able to build immune tolerance to our microbiome or towards our normal flora in order for the T and B cells to avoid attacking our microbiome. And so, again, recall from some of our previous lesson videos that immune tolerance can be defined as the ability for our immune system to distinguish between harmless antigens and harmful antigens. And immune tolerance is very important for preventing autoimmunity or autoimmune diseases, which are characterized by our immune system attacking our own healthy cells.
Now also recall from some of our previous lesson videos that regulatory T cells or Treg cells are important for preventing autoimmunity as well. And that's because these regulatory T cells or Treg cells are going to inhibit the activity of other T cells and prevent those other T cells from targeting the host's microbiome and healthy cells, which, again, we do not want our immune system targeting these cells. Now interestingly enough, it turns out that there are many scientific studies that have shown that early and sufficient exposure to microbes as an infant or child can actually help to increase Treg cell activity or regulatory T cell activity. And that, again, is going to help prevent autoimmune diseases and, can also help to prevent allergies as well.
This directly leads us into what is known as the hygiene hypothesis, which basically states that insufficient exposure to microbes can actually increase a person's risk of developing allergies and autoimmune disorders. And so if we take a look at our image down below, we can get a better understanding of both the hygiene hypothesis and how the microbiome can help to promote immune tolerance. And so notice that this image is broken up into a top half here with a yellow background, and the bottom half here with a pinkish background. And what you'll notice is that on the far left over here, we're showing you an infant or a child here who has exposure to many microbes as an infant. So you can see the diversity of microbes that this infant is exposed to. And, being exposed to so many different microbes at a young age can help to create high regulatory T cell activity. And so notice that there are many regulatory T cells with high activity. And, again, recall that the regulatory T cells like this cartoon is trying to display, they are important for inhibiting the activity of other immune cells to prevent them from attacking the healthy microbiome and healthy cells. And so notice over here, we're showing you these immune system cells that are looking to destroy the microbes. However, the regulatory T cell says stop, only destroy the pathogens. And so it's preventing an immune response towards the healthy microbiome, and helping to redirect the immune cells to target the pathogens only. And so with high Treg cell activity, the immune system is able to build a tolerance for the microbiome. Tolerating the microbiome so that it is not targeted and only attacking the dangerous pathogens.
Now on the bottom left over here, we're showing you an infant or baby that is having exposure to, few very few microbes as an infant. So notice that here we only have very few microbes in comparison to all of these microbes the top baby is exposed to. And so having very few microbe exposure as an infant or child will lead to relatively low regulatory T cell activity or Treg activity. And so notice here that we have a relatively low number of these Treg cells. And so under these conditions, the immune system is going to be at a higher risk of lacking tolerance towards the microbiome. And so that can lead to increased risk of allergies as well as increased risk of autoimmune disorders. And so once again, notice in this cartoon, we're showing you the immune system cells looking to target these microbes. And notice that with low regulatory T cell activity, it will not be able to inhibit these immune cells in the way that they should. And so, notice here that the Treg is saying I can't tell the difference between them anyways, but ultimately it's just that these Treg cell these, immune system cells here will be targeting both the healthy microbiome as well as the pathogens. And again, that could lead to potential allergies and autoimmune disorders.
And so this here concludes our brief lesson on the microbiome and how it can promote immune tolerance. And we also discussed a little bit about the hygiene hypothesis here. And so we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.