Immune System Overview

Let’s do a bit of an overview of the immune system. Having a good understanding of this essential system is the basis of having a good understanding of disease. But first, the immune system can be split into two categories: innate immunity and adaptive immunity.

Innate immunity is your bodies natural response and it’s always ready. Think of it as your sort of standing army, always on hand and ready to go. One of the major components is the largest organ of your body, your skin. The epithelial cells of the skin form a physical barrier that helps prevent pathogens from entering into your system. That’s why wounds get infected, cuts get pussy and rashes get inflamed. It’s also why burn victims are at such an increased risk of infection – a huge portion of their innate immune system is gone! Another example of a sort of barrier component are the gastrointestinal and respiratory tracts. Think of all the fluids and such we have in those tracts to help destroy bacteria. Saliva and tears can lyse (break apart) bacteria. Stomach acid can dissolve pathogens. The respiratory tract has mucus to trap pathogens and cilia to expel them. All these mechanisms are designed to prevent pathogens from even entering the system so we don’t have to fight them on home turf. The last in this first line of defense is the flora (bacteria) that our body already has. You know all about the bacteria that just lives in your gut, right? Well, that’s not the only place we have healthy bacteria, but its a good example. So say a pathogen does make it through your stomach into your large intestine where so much of that microbiome lives. The normal flora presents such a competition for resources that it can actually choke out that infection before it starts.

So, we kind of call everything we’ve talked about the first line of the innate immune response. The other important aspect of innate immunity is that it is non-specific. There is also a second line in this system. This includes phagocytes and neutrophils which cross over from the nearby blood stream to fight the invasion. Macrophages are also present in the tissue to step in. Essentially, they all do the same thing of looking for things that don’t look like they belong. On all of your cell membranes, there are essentially little flags with you on them (called class I MHC). All of your cells wave this flag to show that they belong to you and are part of the team. So, the phagocytes, macrophages and neutrophils will go around checking cells for these flags, almost like bouncers at a bar going around checking IDs. When they run into something that doesn’t have that flag, they destroy them by producing microbicidal substance and causing them to lyse. In the process, cytokines are released which are promote inflammation of the infected tissue and enhance the response. The complement system is also activated which, long story short, creates Membrane Attack Complexes that destroy cell walls. Let’s not get into the details of how that works, because that sounds like a fantastic follow up article.

The second division of the immune system is the adaptive immune system. This is something that grows and develops overtime as you are exposed to new and different pathogens. This also gets split into two categories – the humoral (antibody mediated) and cellular (cell mediated) responses or, if you like, B cells and T cells. The humoral response, or antibody mediated, is essentially how antibodies provide protection in the blood, muscoa and tissues. Cell mediated is all about how T cells split into Helper T cells and Cytotoxic T cells to provide additional a defense against microbes that are intracellular. The specific action of T cells and B cells are both in depth topics, so we’ll reserve the bulk of it for their own articles. Man, this one article has already spawned three spin offs. The nutshell is that the adaptive immune cells are at some point presented with pathogens of different types. They then remember what they look like for future reference. Down the road, if they show up again, these memory cells know how to take them out and enact the proliferation of cells trained to do just that. An understanding of this is how vaccines work and how we build our immune system. It also informs the parent as to why they should allow their kid to get dirty and why over sanitizing everything is actually a bad thing.

We’ll leave it there for now, but look for future articles on this topic soon!

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