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The immune system is what protects all of us from diseases caused by bacteria, virus, fungi and parasites – also known as pathogens.
In order to fulfil this difficult task, the body has developed several sophisticated layers to defend itself. Broadly, we can distinguish between the so-called innate immune defence, which is made up of components that can attack and protect you from pathogens right away and the adaptive immune defence, which takes longer to kick in but is much more specific to the pathogen causing the attack on the body, leading to a more efficient killing of the intruder.
In detail, we have the following lines of defence a pathogen needs to pass before it can make us ill:
- Physical barriers:
The very first lines of defence are actual physical barriers such as your skin. Some of these barriers also secrete chemical substances, which kill pathogens. Examples for this are the low pH in the stomach and enzymes such as lysozyme found in spit and sweat that can destroy bacteria before they can enter the body.
- Cells of the innate immune system
If a pathogen breaches the first barrier, for example via a cut in the skin – leukocytes come to the rescue. These cells are part of the innate immune response and recognise a wide range of structures found on many different pathogens – making them unspecific yet fast responders. In most cases monocytes, macrophages and neutrophils are the first cell types to spot the invading pathogen – and they have a simple solution: They eat it!
But not just that – monocytes, macrophages and neutrophils also produce messenger molecules (called cytokines), which recruit more cells in a process called inflammation. The cells then kill the invaders by both eating them and releasing substances or molecules that will kill the pathogens.
- Other parts of the innate immune system
But the innate immune system does not just consist of cells – there are also non-cellular components in our blood ready to fend of pathogens. The most important is called the complement system, a cascade involving more than 20 molecules. Once activate the cascade can poke holes into the pathogen leading to the destruction of it. In addition to this, some molecules of the cascade function as cytokines recruiting leukocytes to the site of infection.
Can you see the common pattern? All parts of the immune system work together – calling for HELP! in form of more cells and molecules to kill the pathogen and protect the body.
- Adaptive immune system
In case the innate immune system fails to kill all the invading pathogens, the adaptive immune system steps in, in form of lymphocytes called T cells and B cells. These cells are specific to the invading pathogen – each T and B cell will only step into action when seeing its specific antigen. Antigens are molecules only present in the one bacteria or virus that the T or B cell is specific for. You can image this like a puzzle – only some pieces will fit together!
Any given antigen will only activate a small number of specific T cells.
To make this even more complicated T cells do not recognise pathogens directly but rather via so-called antigen-presenting cells. But don’t worry; this is just a different name for some of the innate immune cells we talked about earlier. If you remember these cells eat pathogens to kill them, but they also process their “food” and present small parts of it on their surface to show the lymphocytes what they have eaten and thereby what pathogen is attacking the body.
Once activated T cells can either kill the pathogens via toxic substance release or further stimulate inflammation by producing messenger molecules.
Activated B cells produce so-called antibodies, small molecules that can neutralise bacterial toxins, preventing damage to the healthy tissue and mark pathogens for destruction by macrophages as well as the complement system.
You may now wonder why this specific response isn’t available right away to support the innate immune response. This is simply due to space. Each lymphocyte is specific for one antigen, so to allow us to have B and T cells covering millions of different antigens, each specific B or T cells only exists in limited number. Only once they encounter their antigen, the cells divide in order to produce a larger number of specific B and T cells and allow an efficient response! And this takes – around 5-7 days.
What happens when the invader is eliminated?
Most of the immune cells involved in the destruction of the invading pathogen, are newly formed by division of existing cells after their initial contact with the invader. So when they have done their job they die, to make room for other cells and to allow the recognition of possible new threats.
But not all cells die, a small proportion of T and B cells persist and become so-called memory cells. These cells are highly specialised and allow a quicker and more effective response in case of re-infection with the same pathogen.
These are the very basics of your immune system and we will go into more detail in future blog posts. However if you have any questions, let us know and we will do our best to answer them!