Blood & the immune system:
The immune system is essential for our survival, without it our bodies would be open to attack from bacteria, viruses, parasites and more. It is made of up of organs, cells and chemicals that fight infection. It can distinguish our tissue from foreign tissue i.e. self from non-self. Dead and faulty cells are also recognised and cleared by the immune system [1-2].
The main parts of the immune system are white blood cells, antibodies, the complement system, the lymphatic system, the spleen, the thymus and the bone marrow. These all work as part of our immune system to actively fight infection [1-3].
To distinguish between self and non-self the immune system detects certain proteins on the surface of cells. It learns to ignore its own proteins from an early stage. An antigen is any substance that can initiate an immune response. It’s worth noting that Antigen is short for ‘antibody generators’ [1-3].
The blood is our main transport system throughout the body so naturally it is heavily involved in our immune system.
White blood cells (aka leukocytes):
WBCs circulate in the body in blood vessels and the lymphatic vessels that parallel the veins and arteries. WBCs are on constant patrol, looking out for pathogens. When they find a foreign pathogen they begin to multiply and send out signals to other cells to do the same [1-3].
WBCs are stored in different places in the body – these are referred to as lymphoid organs [1, 3]:
- Bone marrow
- Lymph nodes
Two types of leukocyte [1, 3]:
- Phagocyte – neutrophils, monocytes, macrophages and mast cells
- Lymphocyte – B-lymphocytes & T-lymphocytes (also called B-cells and T-cells)
Phagocytes surround and absorb pathogens and break them down, whereas lymphocytes help our bodies to remember previous pathogens and recognise them if they attack again [1, 3].
- Spot the antigen and begin secreting antibodies
- Antibodies are proteins that lock onto specific antigens
- Each B cell makes one specific antibody
Antibodies are part of a family of chemicals called immunoglobulins (Ig). Immunoglobulins play many roles in the immune system [1-4]:
- IgG – marks microbes so other cells can recognise and attack
- IgM – kills bacteria
- IgA – found in fluids such as tears and salvia
- IgE – protects against parasite and is also responsible for allergies
- IgD – stays bound to B cells, helping to initiate the immune response
Antibodies lock on to antigens, they don’t kill antigens. They mark the antigens to be killed by other cells such as phagocytes.
- T Helper cells (Th cells) – orchestrate the immune response. Some stimulate B cells, some communicate with other cells, while some attract more T cells or phagocytes.
- T Killer cells (cytotoxic T cells)– they attack other cells and useful for fighting viruses. They recognise small parts of the virus on the outside of infected cells and destroy the already infected cells.
Immunity is the capability of our bodies to resist harmful pathogens. Everyones immune system is different but it considered to become stronger during adulthood as we are likely to have been exposed to more pathogens and as a result, more immunity.
Once an antibody is produced a copy then remains in the body so that if the same antigen appears again it can be dealt with much more quickly.
There are three types of immunity in humans called innate, adaptive and passive [1-3].
Our innate immune system is with us from birth, its response is more general and non-specific but it protects us from foreign invaders from day one. The innate immune response is the first rapid response to a foreign invader, when this system recognises a pathogen it goes into action immediately. The cells of the innate immune system called phagocytes, surround and engulf the pathogen where it is then killed. The innate immune system includes external barriers such as the skin and mucous membranes. If a pathogen manages to get past the innate immune system, the adaptive immune response kicks in [1-2, 6-7].
The adaptive immune system, with help from the innate system, produces antibodies to protect us from specific invaders. If we are exposed to diseases or get vaccinated then we build up a bank of antibodies to different pathogens. The adaptive immune system “remembers” previous pathogens and can respond quickly if they return [1-2, 6-7].
Passive immunity is “borrowed” from another source and does not last. For example, a baby receives antibodies from the mother through the placenta before birth and in breast milk after birth. This passive immunity protects the baby from some infections during the early years of their life. These antibodies disappear between ages 6 and 12 months [1, 7].
Our bodies have other important defence mechanisms such as :
- Digestive Tract
- Other defences: bodily fluids like skin oil, saliva and tears (all contain bacterial enzymes that help reduce the risk of infection)
Another important part of our immune response is the inflammatory response (inflammation). Inflammation occurs when tissues are damaged by bacteria, trauma, toxins, heat or any other cause. Damaged cells release chemicals which include, histamine, bradykinin and prostaglandins. These chemicals can lead to blood vessel leakage into the tissues, which in turn causes swelling. This helps to isolate the foreign substance from further contact with other body tissues .
The chemicals also attract phagocytes that “eat” the germs, dead or damaged cells. This process is known as phagocytosis .
The complement system is another key part of our immune system that enhances or complements the ability of antibodies and phagocytes to clear microbes and damaged cells from our bodies. The complement system also promotes inflammation and attacks the cell membranes of pathogens.
Blood coagulation, complement and the fibrinolytic systems are highly interactive, when one system becomes dysregulated, activation of the others are altered and processes such as thrombosis and chronic inflammation can occur. This is why in cases like sepsis (infection and inflammation) we see severe changes in the blood coagulation cascade whereby it becomes activated resulting in disseminated intravascular coagulation (DIC) resulting in a higher mortality rate [9-11].
Platelets were originally deemed to only play a role in prevention of bleeding but in the last decade or so they have been recognised for their activities independent of thrombosis. Platelets are capable of storing and releasing bioactive mediators and express a wide range of functional immunoreceptors  .
Platelets in innate immunity:
Platelets have the ability to release inflammatory and bioactive molecules stored within granules (dense and alpha granules) or that can synthesised upon activation. These mediators attract and modulate effector cells of the innate immune system. Platelets themselves also demonstrate direct effector function and therefore are regarded as cells in the innate immune system [12-13].
Platelets are the first to detect endothelial injury and microbial pathogens as they gain access or invade the bloodstream or tissues. Injury to the endothelium exposes collagen and other membrane proteins allowing platelets to adhere. Stable adhesion to collagen leads to platelet aggregation and causes the release of platelet agonists such as ADP, thrombin & vWF. This leads to the platelet activation and further recruitment of platelet to the site of injury and infection. This serves two purposes, the first being haemostasis to stop blood loss and the second being frontline defence against microbial infection. Also, platelets express a number of chemokine receptors which detect signals for all four classes of chemokine generated at sites of infection. This leads to rapid accumulation of platelets to the site of injury & infection [12-13].
Platelets have been known to influence the innate immune response through regulation of macrophages, neutrophils and dendritic cells. They can also interact with bacteria, viruses, fungi and protozoa, they also demonstrate anti-microbial functions too! Growing evidence also suggests that platelets play a role in adaptive immunity, shaping the immune response . Despite being anucleate cells, i.e. cells without a nucleus, platelets play vast and surprising roles in both haemostasis and immunity, which is something I might explore in a separate blog post – is there anything our platelets can’t do?!
As you can see our immune system is an extremely important and complex system, without it we wouldn’t be able to fight against pathogens that we come into contact with regularly. This has been an overview of blood and the immune system, I hope you enjoyed this post!
Let me know in the comments if you know any interesting facts about our immune system – I’d love to learn more about our fascinating immune system!
Thank you for reading!
See you in the next one,
 Medicalnewstoday.com. 2020. The Immune System: Cells, Tissues, Function, And Disease. [online] Available at: <https://www.medicalnewstoday.com/articles/320101>
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 TeachMePhysiology. 2020. T Cells – Production Of T Cells – Types Of T Cells – Teachmephysiology. [online] Available at: <https://teachmephysiology.com/immune-system/cells-immune-system/t-cells/>
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 Immunology.org. 2020. Phagocytosis | British Society For Immunology. [online] Available at: <https://www.immunology.org/public-information/bitesized-immunology/systems-and-processes/phagocytosis>
 Institute of Immunity and Transplantation. 2020. Coagulation & Inflammation. [online] Available at: <https://www.ucl.ac.uk/immunity-transplantation/research/gene-therapy/coagulation-inflammation>
 Oikonomopoulou, K., Ricklin, D., Ward, P. and Lambris, J., 2011. Interactions between coagulation and complement—their role in inflammation. Seminars in Immunopathology, 34(1), pp.151-165.
 Simmons, J. and Pittet, J., 2020. The Coagulopathy Of Acute Sepsis.
 Ali, R.A., Wuescher, L. M., & Worth, R. G. 2015. Platelets: essential components of the immune systems. Current trends in immunology, 16, 65-78.
 Semple, J., Italiano, J. and Freedman, J., 2011. Platelets and the immune continuum. Nature Reviews Immunology, 11(4), pp.264-274.