Background information
Not enough vitamin B12? These groups are at risk
by Moritz Weinstock
Come to think of it, how does our immune system actually work?
20/3/2023
Translation: Veronica Bielawski
Each of us has a sophisticated defence system working around the clock to protect us from attackers. Find out what killer cells are all about and why some of our immune cells self-destruct to protect us.
There’s no shortage of danger in life. And if you didn’t have a sophisticated defence system working 24/7, you’d be at the mercy of millions and millions of attackers, pollutants and toxins. Needless to say, it’s a good thing that you have this protection against bacteria, viruses, fungi and other threats. In fact, many attackers slip in and out unbeknownst to you. Your immune system fights invading pathogens before they become a real problem. Unfortunately, this isn’t a fail-proof system. Things like vaccines, for example, support your immune system by helping it recognise dangerous viruses or bacteria it isn’t familiar with. If you then encounter these pathogens, your immune system is already attuned to them and can defend itself more quickly.
Your immune system consists of a network of cells, tissues and organs that work together to protect your body from invading pathogens. In addition to the skin and mucous membranes, this includes a large number of defence cells. Immune organs include the thymus (were T cells are made), bone marrow (where immune cells are produced), the spleen, lymph nodes and the lymphoid tissue of mucous membranes (e.g. your tonsils).
Your immune system can be divided into the innate immune system (non-specific defence) and the adaptive immune system (specific defence). These two systems are closely linked, but perform different tasks.
The innate immune system: the all-rounder
Your innate immune system, that is, non-specific defence, is your first line of defence against pathogens. It includes physical barriers such as your skin and mucous membranes. You can think of it as a castle wall – there to prevent any enemies from entering in the first place. In addition, your saliva, tears and mucus help keep the mucosal surfaces clean and contain special proteins that kill bacteria. Lysozyme, for example, acts by breaking down the cell walls of various bacteria.
Phagocytes and killer cells: the tacklers
In addition to these barriers and secretions, you have special cells permanently on guard. Pathogens end up penetrating your body often enough despite all the defence mechanisms – for example, if you have a cut and your skin can no longer perform its protective function. That’s when your cellular defence mechanisms come into play. Enter: phagocytes and killer (K) cells.
Phagocytes work by ingesting enemy cells. Phagocytes are white blood cells (leukocytes) that can leave the bloodstream and migrate into infected tissue, where they have a go at the intruders. Once they’ve had their fill, they self-destruct. Their remains – along with the destroyed, and therefore harmless, pathogens – can then be safely transported away.
In addition, there are the killer (K) cells, which include so-called cytotoxic (i.e. lethal to cells) T cells and natural killer (NK) cells. The name says it all: killer cells recognise your body’s cells that have been attacked by pathogens (or even cancer) and attack the plasma membrane, i.e. the cell’s outer layer, causing the infected cells to burst [1].
The adaptive immune system: a memory mogul
The adaptive immune system (specific defence) is a sophisticated defence system that recognises – and remembers – specific pathogens. The two most important cell types involved are T lymphocytes and B lymphocytes. These cells can produce antibodies (specific proteins) that target specific pathogens.
The antibodies use special surface structures called antigens to recognise hostile viruses, bacteria, fungi as well as pollen and transplanted tissue, among other things. The term antigen means antibody generator; antigens activate lymphocytes, which then produce antibodies.
Fight off, form memories
These activated cells then divide. Each lymphocyte that binds to the specific antigen forms two copies of itself. So-called effector cells are formed, which fight the antigen. As a result, these cells target the attacker right at the moment of infection. In addition, your body prepares for any future attacks by the same enemy by forming memory cells. In short, you become immune to the pathogen. Your body remembers it and will quickly recognise it as dangerous during the next infection and start fighting it. In fact, memory cells will often remember the antigen years later or even for a lifetime. This is also the process vaccines rely on. Namely, your immune system is presented with pathogens (fragmented or in a form that’s been rendered harmless). That way, it can store them as a threat in order to react quickly to an infection with the active pathogens.
Unfortunately, not infallible
Your immune system constantly monitors your body for potential threats and is able to respond quickly to new invaders. But – like anything in the world – it’s not perfect. Sometimes, it can malfunction. If it mistakenly attacks your body’s own tissue, this can lead to autoimmune diseases. What’s more, certain pathogens can evade the immune system, such as the human immunodeficiency virus (HIV), which causes AIDS. HIV infects specific T cells (CD4-T), which become damaged and degraded, weakening your immune system. In addition, these viruses are constantly changing, making the immune response more difficult. And they can permanently hide in reservoirs within the body, where they’re difficult for drugs or your immune system to reach.
Did you know that ...?
Your microbiome (the collection of microorganisms that live in and on your body) also plays a critical role in shaping and supporting your immune system. In 2009, for example, researchers showed that the gut microbiome helps train the immune system to better recognise and respond to pathogens [2].
Sources:
[1] Akbari, O., & De Groot, A. S. (2015). Innate immune recognition of pathogens and danger signals. Cold Spring Harbor perspectives in medicine, 5(11), a017921.
[2] Round, J. L., & Mazmanian, S. K. (2009). Inducible Foxp3+ regulatory T-cell development by a commensal bacterium of the intestinal microbiota. Proceedings of the National Academy of Sciences, 106(35), 12033-12038.
Header image: pexels/Polina Tankilevitch
Science editor and biologist. I love animals and am fascinated by plants, their abilities and everything you can do with them. That's why my favourite place is always outside - somewhere in nature, preferably in my wild garden.
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