Most of us have heard of the immune system. “Oh, I’m a bit tired today, my immune system is run down”. But how many of us know exactly what an immune system is. Or what is does! This post is going to go through it step by step and hopefully by the end we’ll all have a better idea. I have to say immunology (the study of the immune system) was never my strongest subject. We’re all learning today!
So, first things first. Your “immune system” is not one cell but a collection of many types of cells that work to together to protect your body. I will be going through each cell type and their function. Just as a note “white blood cells” are another name for your immune system. In your blood you have red blood cells – which transfer oxygen around the body, plasma – which is the liquid part of blood that carries all sorts of things like hormones, glucose etc. and white blood cells – which are your immune cells.
Also, white blood cells are called leukocytes. (This is why I find immunology difficult…too much to remember).
So where are your immune cells made? The bone marrow! Immature immune cells (basically immune cells that haven’t learnt what they should do yet) are made in the bone marrow. They either do basic schooling there to learn what to do or they go to fancy school in the pituitary gland to get a full education. More on this later.
There are two types of immune system:
Your innate immune system is like a security guard walking around a museum. They are there to keep an eye on all the cells and deal with any small disturbances (such as the odd bacterial/viral cell). The innate immune system is made up of a number of cells. They are constantly going around your body patrolling. Innate immune cells can recruit other cells to the site of invasion and can also raise the alarm for a more comprehensive immune response. They do this by releasing signals called “cytokines”.
Here’s the list:
Mast cells: these cells reside in connective tissue and take part in wound healing.
Phagocytes (eating cells)
Natural killer cells: these cells kill normal cells that have been taken over by virus etc.
Gamma-delta T-cells (γδ T-cells): these cells are both innate and adaptive. Not much is known about these cells. Fun fact a PhD I know (Marina) studies γδ T-cells for cancer therapy. She took a blood sample from me and my γδ T-cells are really good at killing cancer cells.
A note on the immune system:
The typically immune response takes about 4-7 days to ramp up when the immune system has not seen the threat before. This is why when you have the flu it takes a full week or more to get over it. It’s also why you feel shocking. For the first few days while your adaptive immune system is metaphorically getting themselves together, the invader is running riot through your body.
When your immune system has seen the threat before (in the case of vaccines or previous exposure) your immune response it way faster. It will typically take 3 days for the immune system to kick in.
Fun fact! There is a layer of fat across your abdomen called the omentum. It kinda looks like an apron sitting over all your vital organs. The omentum is full of immune cells (all kinds) all the time. When you get major surgery, the surgeon will wrap the area with omentum to help with healing. It’s probably one of the most interesting areas of fat in your body (to me anyway). Another fun fact! Surgeries like liposuction tend to remove “good” fat (like what the omentum is) while leaving “bad fat”.
Your adaptive immune system is more complicated. Your adaptive immune system is not always active. They don’t patrol around at all time. In fact, your adaptive immune system is like S.W.A.T. It is activated when there is a serious threat and provides the big guns (as it were) to deal with the issue. Your adaptive immune cells (called T-cells and B-cells) are made in the bone marrow. They then travel to the pituitary gland where they are trained. This is where antibodies and antigens come in.
Antibodies are on immune cells whereas antigens are on every other type of cell. It acts like a hand shake. A cell will “put out their hand - antigen” and the immune cell will “take the hand - antibody”. If the antigen is recognised by the antibody nothing happens, and the immune cell moves on. If the antigen is NOT recognised by the antibody, then the immune system responds. Innate immune cells can present the antigens that were on the invaders they engulfed and this attracts adaptive immune cells to the site of invasion.
But the immune cells need to learn these handshakes.
The best example is blood types. There are many blood types – A+, A-, B+, B-, AB+, AB-, O+ and O-. These letters are antigens. So in a person with A+ blood type it means that on their red blood cells there is an antigen called A, whereas a person with A- doesn’t have the antigen called A. So in normal circumstances an A+ person will only have A+ blood cells. The immune system sees this and is fine. But if that person got a transfusion with say A- blood, the immune system will try to do the “handshake” but the A- blood cells won’t be able to. This is when the immune system goes into defence mode, destroying all the A- blood cells. This is called graft vs host disease and happens a lot with blood transfusions and organ donation.
Another example is vaccines. A vaccine is the antigen of the virus without the harmful parts. It allows the immune system to learn that the “handshake” is bad, which the adaptive immune system remembers. So if the real thing comes along the adaptive immune system can jump in immediately.
Here’s the list (lymphocytes):
Interesting fact: your immune system will also get rid of cells that have “gone bad”. Basically, if they are displaying abnormal behaviour, they can’t do the handshake, then they’re outta here!
Another interesting fact (I’m full of them), a fever is a tactic of the immune system. Most cell systems are very sensitive to temperature. A fever is a case of “sacrifice a few to save the many”. Your body temperature soars above the normal temperature for viruses, bacteria and your own cells. This helps kill off the invaders. You will lose normal cells and immune cells (and feel horrendous in the process) but the threat is gone! So yay!
The fun (?) part. Diseases associated with the immune system.
Numero uno! Allergies! When you have an allergic reaction, it is your immune responding to what it considers harmful. Now I really don’t know what is hugely harmful about pollen, egg yolk, latex, avocado and chestnuts (some of my wonderful allergies) but sure look. That’s what my immune system thinks. Now the degree of allergic response can go from mild (hives, vomiting, flu-like symptoms etc.) to life threatening (anaphylactic shock and death).
Second! Auto-immune diseases! These really is what it says on the tin. You know we talked about the handshake your adaptive immune system learns to distinguish your cells from invading cells? Well in the case of auto-immune diseases your adaptive immune cells are…well…not very good at learning. They attack your own cells.
Third! Cancer! Namely leukaemia (AML, CML and CLL), non-Hodgkin lymphoma, Hodgkin lymphoma, multiple myeloma. I will do a more comprehensive article about blood cancers, but these are all cancers of different immune cells.
Fourth! Cancer! Oh yes, your immune system can play a role in ALL cancers. Initially your immune system won’t recognise that cancer cells are bad and the cancer grows. Eventually the immune system catches on and tries to get rid of the cancer. But hey! Cancer is smart and uses Tregs and other regulatory immune cells (like tumour associated macrophages - TAMs) to calm the immune response. They can also change what antigens they present. And then they infiltrate the immune system (like spies) and turn the immune system within the tumour to their side. This helps the cancer grow, while also telling the rest of immune system to “move on, nothing to see here”.
The best part! We can use the immune system to fight back! Take that cancer!
This is what we call immunotherapy. We use the ways cancer manipulates the normal immune system and re-manipulates the now cancer immune system (is re-manipulates a word? Anyway you get me). There are a couple of ways this can be done.
1 – you shut down regulatory T-cells, basically preventing cancer from stopping your immune system
2 – you remove the antigen “handshake” on the cancer cells. This prevents the cancer cell from pretending it’s normal, allowing the immune system to kick in.
The problem with these two methods is it can affect your whole immune system, not just the one targeting cancer. This opens a can of worms in terms of knock on effects on your normal healthy cells. You do not want that!
But there may be a way of killing cancer and sparing your healthy cells. Some of your adaptive immune cells can be taken out of your body (via blood donation) and re-taught how to kill cancer. Specifically, your cancer. The white blood cells can be isolate from the rest of the blood and “primed” to know what handshakes your cancer is presenting. The army of cancer killing immune cells is the grown so there are enough to do the job and put back into your body. These primed immune cells go directly for the cancer, killing it while leaving your healthy cells to rejoice! Throw a parade! This therapy is still in trial phase but provides promise for more effective cancer treatment.
If you have any questions or comments, please don’t hesitate to contact me. If I have gotten anything wrong or missing information, please let me know!! It’s important we all get the correct information.
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My name is Caitriona and I am a PhD student at Imperial College London, UK.