Table of Contents
The human body is constantly exposed to influences from the environment in which pathogens – viruses, bacteria, fungi and parasites – are found. Its only external mechanical protection is the skin and its internal protection is the mucous membranes, such as those of the digestive tract and respiratory tract. Anything that penetrates these mechanical barriers is automatically a threat to the proper functioning of the entire human body.
Bacteria are very happy to use our body’s environment for their own survival, and viruses need our cells to reproduce. By penetrating the inner parts of the body, infections are created which need to be effectively fought against. Fortunately, there is an army just below the external and internal surfaces of the body ready to protect your body until the last minute of your life. This army is the immune system, which works around the clock, 24 hours a day, 7 days a week.
What is the immune system?
The immune system is not a separate organ. It is found everywhere in the body and its main highways are the blood and lymph. It travels through your blood vessels to go wherever there is a danger of breaching the mechanical boundary (skin and mucous membranes) between the outside world and the sensitive interior of the body. Through the blood and lymph, the cells of the immune system reach the immune organs – the thymus, spleen and lymph nodes.
An entire army of immune system cells is born in the bone marrow. White blood cells (leukocytes) represent the individual combat units and, depending on their specialisation, are divided into specific and non-specific immunity. The non-specific immunity includes a variety of proteins and the specific immunity is linked to the non-specific immunity through antibodies.[1-2]
1. Nonspecific (innate) immunity
The first line of defence in the human body is non-specific immunity. This defence is innate and can effectively protect you against the most dangerous pathogens. It consists of cells that are deployed in critical places like machine gun nests. If something that doesn’t belong in the body manages to penetrate, these cells start firing with all the weapons at their disposal.[1]
But to let these cells know when to fire, there are sensors – proteins – scattered around the body. These proteins belong to the so-called complement system. Their job is to stick to bacteria and viruses and pass on the signal to the cells to launch an attack. They recognise carbohydrates that are found on the surface of viruses and bacteria, but not on the surface of your cells. [3]
The first fighting unit at the site of infection are neutrophils. These cells are most numerous in the blood and are constantly renewing themselves. Their main function is to attack and literally gobble up everything that does not belong in the body. If necessary, they will perform kamikaze – they will burst open and spew their contents onto the battlefield, thus promoting local inflammation and incapacitating the enemy.
The complement system together with the cells of non-specific immunity trigger the inflammatory process. It works like a forest fire to prevent the spread of pathogens. The inflammatory response is non-specific and therefore we perceive it as something unpleasant – even those of your cells that are nearby suffer. [3]
Non-specific immunity includes other cells besides neutrophils: [1]
- macrophages – with neutrophils belong to phagocytes and are involved in “gobbling up” pathogens
- dendritic cells – link non-specific and specific immunity by showing antigens to other cells
- mast cells – involved in the immune response and secrete substances such as histamine (in allergic reactions)
- basophils – they are the least abundant in the blood but trigger a very strong (allergic) reaction
- eosinophils – similar to basophils, but also involved in protection against fungi and parasites
- NK (natural killer) cells – are equipped with enzymes that enable them to kill virus-infected cells or tumour cells
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2. Specific (acquired) immunity
Specific immunity inherently includes cells that remember the enemy and, thanks to this memory, can produce specific antibodies. After a successful attack by non-specific immunity, pathogens are shredded to pieces. These pieces of pathogens are exposed on the surface of so-called antigen-presenting cells and shown to the cells of specific immunity, which learn to recognise these pieces. After encountering the bacterium or virus again, one’s immunity already knows the enemy and is ready to produce a very powerful weapon – antibodies. [2]
Antibodies are produced by B-lymphocytes so that they recognise antigens on the surface of pathogens and the attack is directed only at this. Antibodies are like a smoking gun that indicate where to bombard. This smokescreen is also used by the complement protein system to be able to attack specifically, leading to more efficient engulfment of pathogens. At the same time, they are used by cells of specific immunity, which, when recognised by an antibody , kill the infected cell to prevent the infection from spreading.
Specific immunity includes in particular: [2]
- T-lymphocytes – they are divided into several subtypes and their main function is to recognise the pathogen labelled with antibodies and destroy it
- CD8+ T-lymphocytes are cytotoxic – they kill infected cells
- CD4+ T-lymphocytes are helpers – they activate B-lymphocytes and CD8+ cytotoxic lymphocytes
- B-lymphocytes – are responsible for the production of antibodies based on antigens that have been “shown” to them
How does the fight against a pathogen occur?
- If hostile forces (viruses or bacteria) manage to penetrate through the skin into the body, they are in for an unpleasant surprise in the form of proteins that immediately react to this entry. Under ideal conditions, this response is rapid and effective.
- After the initial recognition of the pathogen, several events take place that lead to the marking of the enemy for cells of non-specific immunity, eventually leading to its total destruction. However, bacteria and viruses have evolved mechanisms to resist these attacks.
- Non-specific immunity cells come next. They shoot indiscriminately at anything that has been marked as an enemy. They produce substances that trigger an inflammatory reaction – a wildfire that destroys everything around it.
- The enemy is captured – engulfed by cells. Its identifiers (antigens) are dissected and shown to the cells of specific immunity.
- During ongoing battles (disease), there is also the production of antibodies, which arrive on the battlefield a little later, but focus their attack directly on the enemy.
- The inflammatory reaction, together with the “gobbling up”, destruction of cell walls and subsequent production of antibodies, leads (ideally) to neutralisation of the enemy.
The biggest advantage is that the cells of specific immunity remember what the enemy looks like and can thus coordinate an attack in the event of repeated infection.
Under very specific circumstances, antibodies can be formed against the body’s own proteins. In this case, the immune system is confused and a civil war – autoimmune diseases – takes place. [4]
The immune system is constantly ready to protect the body from infections, but it also keeps order in its own ranks. It is involved in recognising not only infected, but also old and potentially cancerous cells.
Immunisations teach immunity
Vaccination is the best infection prevention we have. It uses the immune system’s ability to learn and remember pathogens. This learning, however, does not require an infection, which brings with it the disadvantage of a disease in which immunity is firing on all cylinders.
Vaccination involves exposing already digested pathogens (antigens) to the immune system so that it produces antibodies before the pathogen enters the body. It’s like a pre-mission briefing, where the cells of the immune system know the enemy before it appears so they can precisely target their attack. [5]
The principle is simple:
- A vaccine containing a piece of a virus or bacteria (antigen) is injected into the body (most often into a muscle).
- The cells of the immune system take these pieces and show them to others.
- The production of antibodies against the injected antigen is ongoing.
- In the event of infection, the most accurate and effective weapon against the enemy is thus ready.
There are several ways to show antigens to the immune system using immunisation, and the following can be used as vaccines:
- a weakened or non-living virus
- non-living bacteria
- whole antigens or parts thereof
- vectors that contain information on how to make the antigen. The cells receive this vector, make the antigen and show it to the immune system. Such vaccines were AstraZeneca’s COVID-19 vaccines or Russia’s Sputnik V
- mRNA encoding an antigen or part of an antigen, which is used by cells to make the antigen and present it to the immune system. These vaccines were used in the COVID-19 vaccines from Pfizer and Moderna
How can you boost your immunity?
The immune system is ready to constantly defend the body from incoming enemies. However, we can improve its effectiveness by getting enough sleep, when the cells of the immune system are recycled and new, fresh fighting units are created. As with everything, quality and varied diet, rich in protein, minerals or vitamins and less alcohol, will also help. Immunity can also be boosted by cold therapy, or conversely by having a sauna.
[1] Hato T, Dagher PC. How the Innate Immune System Senses Trouble and Causes Trouble - doi: 10.2215/CJN.04680514. Epub 2014 Nov 20. PMID: 25414319; PMCID: PMC4527020.
[2] Bonilla, Francisco A.; Oettgen, Hans C. . (2010). Adaptive immunity.- doi:10.1016/j.jaci.2009.09.017
[3] Dunkelberger, J., Song, WC. Complement and its role in innate and adaptive immune responses - https://doi.org/10.1038/cr.2009.139
[4] Wang, Lifeng et al. “Human autoimmune diseases: a comprehensive update.” Journal of internal medicine - doi:10.1111/joim.12395
[5] Pollard, A.J., Bijker, E.M. A guide to vaccinology: from basic principles to new developments - https://doi.org/10.1038/s41577-020-00479-7
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