The First Line of Defence (Innate Immunity): Immunology #4
The first line of defence of the immune system is made up of various host barriers.
The immune system is an amazing but very complex system that works all day, every day, to keep us alive.
By knowing how the immune system functions you will be able to understand how medications and vaccines work, as well as appreciate the healing process (which can sometimes be a pain, literally).
In the last article, The Key Players of the Immune System — Immunology #3 met the white blood cells (leukocytes) — also known as the sensor cells of the innate immune system.
In this article, we’ll go into more detail on the first line of defence, which is the first part of the innate immune system.
Important Terminology
Here is a list of some terms that will come up in this article.
The First Line of Defence: Host Barriers
The immune system is made up of four lines of defence. The first line of defence is part of the innate immune system. Host barriers prevent pathogens from entering the body and taking up residence.
Host barriers include physical barriers, chemical barriers, and microbiological barriers.
Physical Barriers — Skin.
The physical barrier you are probably most aware of is the skin. The outer layer of the skin — the epidermis — is made up of tightly packed dead cells. These cells are densely packed with a protein called keratin. Keratin is very tough and is resistant to chemical breakdown by bacterial enzymes. This forms a physical barrier to prevent pathogens from entering your body.
Additionally, the outer layer of skin is continuously shed and replaced by underlying skin cells. This also protects the body from pathogens as when the skin cells are shed, any bacteria that may be on the outer layer of your skin will be shed along with it.
Your skin cells also produce peptides called defensins which are essentially natural antibiotics. Defensins kill pathogens that are not supposed to be on your skin.
Another protective mechanism of the skin is the ‘acid mantle’. The acid mantle is a slightly acidic film on the surface of your skin. It makes your skin a sub-optimal environment for pathogenic growth, so it inhibits large colonies of bacteria from residing on your skin.
Physical Barriers — Mucous Membranes.
Mucous membranes line some organs and cavities in your body such as the nose, mouth, lungs, and urinary and digestive tracts.
Mucous membranes are made up of a layer of cells that secrete mucus. This mucus traps pathogens which are then pushed out of the body by hair-like appendages called cilia. Cilia move the mucus and trapped pathogens up towards the nose and mouth where they can be coughed or sneezed out.
Chemical Barriers.
Lysozymes are antimicrobial peptides that are found in tears, sweat, saliva, and mucus. These peptides break down the cell walls of bacteria, causing them to die.
Additionally, the stomach contains gastric acid which helps to digest food. This acidity also prevents pathogens from residing in your digestive tract as many pathogens do not like such an acidic environment.
The acidity of our digestive tract changes rapidly while you (or more likely, a pathogen) travel through it. While the stomach is quite acidic, the pH is almost neutral in the duodenum (the first part of the small intestine). Then, it becomes slightly alkaline at the other end of the small intestine and drops to become acidic again where the small and large intestines meet. The changes in pH are difficult for the pathogen to cope with if it gets into the digestive system.
Microbiological Barriers.
Although many think of bacteria as being ‘bad’, there are some bacteria that have made a friendly pact with your body. Your body has resident microbiota which are bacteria that are allowed to live on your skin and in your gut.
The resident microbiota is essential to your health in many ways and is often an indicator of your health. One important way that resident bacteria help to keep you healthy is by simply taking up space. By occupying certain areas in your body and digesting the nutrients available to them, they make it rather difficult for pathogens to come in and set up camp.
Conclusion
The immune system is made up of four lines of defence. The first line of defence is the host barriers. Host barriers include:
- Physical barriers — skin and mucous membranes
- Chemical barriers — lysozymes, stomach acid, and rapid pH changes
- Microbiological barriers — resident microbiota
In the next article, we’ll look at the second line of defence and how the sensor cells actually protect you and your body.
