The immune system is the body’s main way of detecting and fighting off infections, and is closely linked to how sepsis affects the body. Each of the links below takes you to a different part of how the immune system works.
Part 1: Infections
Infections happen when harmful germs enter a person’s body and cause a reaction. There are many different types of germs, and these include bacteria, viruses, fungi and parasites.
The human body is exposed to many different germs all the time, but it is usually able to stop the germs from causing an infection with the immune system.
Infections can happen in any body system from head to toe, and cause a reaction. The most common infections are chest infections (also known as pneumonia or lower respiratory tract infections), urinary tract infections, and skin infections (also known as cellulitis).
Infections can cause many different reactions in the body. Some reactions are related to where the infections are (for example, burning sensations when passing urine might suggest that a person has a urine infection); but some reactions are not so specific, such as feeling faint due to low blood pressure. These non-specific reactions make it challenging for healthcare professionals to figure out that an infection is happening in the first place (there are many causes of feeling faint, for example), where the infection is and how to treat it.
Part 2: The immune system
The immune system is made up of different organs, cells, and proteins that work together. The cells and chemicals of the immune system patrol the blood stream, live in lymph nodes and the spleen and other organs, ready to perform their functions.
The immune system is the body’s main protection against infection, and it has 3 main roles:
- To detect any germs or particles that could cause the body harm and try to stop an infection
- If an infection occurs, to destroy the germs or particles that have caused it
- To remember the germs and particles that have caused an infection and have a system in place to react quickly next time.
Part 3: Detecting an infection
The immune system detects an infection by recognising that the outside layer of the germ cells is not the same as the outside layer of the body’s cells. It will look for infections in areas where tissue has been damaged, because areas where the normal cells have been damaged represent easy entry points for germ cells. When the immune system detects a germ cell, it ‘sounds the alarm’ with a process called inflammation.
Inflammation has 3 essential roles in fighting infection:
- Getting more chemicals and cells from the blood stream to the site of infection
- Build a temporary blood clot to seal the area of injury
- Promote healing at the area of injury
The process of inflammation
Before infections
This is a cross section look at a blood vessel (pink circle) with sleepy immune cells that pass through when there are no infections or injuries.
An infection strikes
An area of injury of the blood vessel cells occurs after repeated attacks by the infectious germs, and some of them begin to enter the blood vessel. The sleeping immune cell awakens, recognises the germs and sounds an alarm, getting many more immune cells to the site of injury to fight off the germs.
Leaky walls
All the immune cells are assembled inside the blood vessel, but they are most useful outside fighting the infectious germs before they can get into the blood vessel. To help get the immune cells and chemicals into the right place, the cells produce some special chemicals that makes the wall of the blood vessel looser and ‘leaky’ so that there are more ways for the immune cells to fight the germs outside the blood vessel from all angles.
Plug the hole
Eventually, the immune cells and other cells in the blood vessel such as platelets work together to make a blood clot to place over the area of injury to stop further germs from coming into the blood vessel. The immune cells also produce other chemicals to promote full healing of the injured area.
Part 4: Destroying an infection
The immune system can destroy an infection in many ways:
- By ‘swallowing’ the germ with an immune cell (macrophages)
- By making chemicals (cytokines and chemokines) that can kill the germs
- By making chemicals (cytokines and chemokines) to alert other immune cells that can specifically destroy the germs, either with special features of the cell itself (cytotoxic T cells), or using specialist chemicals (antibodies)
Methods 1 and 2 are methods performed by the innate immune system, which is not so specialist and is good at destroying some of the more obvious germs trying to cause an infection. Occasionally, some germs manage to avoid detection by the innate immune system, or are detected but cannot be destroyed by the innate immune system alone. That is when the adaptive immune system (the more specialist cells) becomes active.
Part 5: Remembering the infection
The adaptive immune system has 2 major types of cells: B lymphocytes (or B cells) and T lymphocytes (or T cells).
Recognising germs
B and T cells can recognise germs that may cause harm to the body. They do this using different shaped molecules on their outsides, using a method almost like jigsaw puzzles. The molecules on the outside of a B cell can only fit with a complementary molecule on the outside of the germ.
B cell actions
B cells are the main producer of antibodies in the body. The antibodies are usually the same shape as the molecules on the outside of the B cell. Just like how a B cell can recognise a specific germ, it can also make antibodies against that germ. During an infection, the B cell can clone itself to make many more B cells that can make more antibodies against that germ. The antibodies will surround the germ and mark it for destruction by the other immune cells.
T cell actions
T cells have 2 abilities. Firstly, they can detect germs using their outside molecules and are also able to detect germs that are hiding inside body cells.
Secondly, T cells are able to interact with the immune cells in the body that have destroyed germs that they have encountered before. The immune cells give the T cell a piece of material from the germ they have captured and the T cell displays it on its outside.
This is to show all the immune cells what the germ looks like so they can recognise a similar germ the next time it tries to cause an infection. This is how the immune system remembers past infections and prevents future ones.
Part 6: Genes in the immune system
Genes play a big part in how the immune system works and fights infection.
Genetic material starts with each person’s deoxyribonucleic acid (DNA). DNA has 2 strands – one from each parent (see picture below). Each cell in the human body contains DNA right in the middle and the DNA is responsible for what each cell looks like and what each cell does.
DNA is very complex – the strands are very long and full of information. It would be too much work for the cell to unravel all of it to look at every time it is needed – a bit like having to read a 10,000 page manual each time you want to use the microwave to defrost something. Instead, the body unravels one short segment of information every time the body needs it, and separates the 2 strands into 1.
This segment of the DNA is then copied by the cell. The copied segment of genetic material is called ribonucleic acid, or RNA. This process is a bit like finding the correct page in the manual for defrosting and photocopying it for use.
The RNA is then matched up with proteins by the cell, forming a chain of proteins called polypeptides. The polypeptides are a bit like the writing on the page of the manual, telling the cell what to do.
The polypeptide chains have lots of useful functions. They are sent to all parts of the cell, changing the shape of the cell, starting or stopping chemical production, building new parts of the cell and telling different parts of the cell what to do.
In the immune system, the body’s genes tell the immune cells what to do if they spot an infection, how to make chemicals to attract attention of other immune cells, how to make chemicals to destroy germs, how to ‘swallow’ germs and how to make antibodies and store memories.
Sepsis Science 