How the immune system works

It’s our immune system that protects us from germs to stop us from getting ill - and in most people it does a good job. However, in some cases there can be problems with the immune system which can lead to autoimmune conditions such as inflammatory bowel disease (IBD).

To help understand what an autoimmune condition is it’s important to understand more about the immune system and how it works.

What is the immune system?

The immune system is the body’s defence against germs and is essential for survival. It is made up of a network of cells, tissue and organs that work together to keep our bodies healthy. It is the second most complex system in the human body - with the nervous system being the first.

The immune system has three main functions:

  1. Attacking pathogens (such as bacteria, viruses, parasites and fungi) and removing them from the body to prevent us from getting ill
  2. Recognising and netralising harmful substances from the environment
  3. Fighting against the body’s own cells that have changed due to an illness - such as cancerous cells.
The immune system is incredibly clever and recognises the cells that make up our bodies. When it comes across something it doesn’t recognise it will try to get rid of it by attacking it. Our white blood cells (leukocytes) play an important role in doing this. They work to seek out and destroy any germs which may cause the body harm. They are made and stored in various places around the body. These include the thymus, spleen and bone marrow. They also live in lymphoid tissue (mainly lymph nodes) throughout the body.

The white blood cells circulate through the body between organs and nodes seeking out germs and other foreign bodies. They travel around in your blood stream via lymphatic vessels (tubes) and blood vessels.

You make about 1000 million white blood cells a day!

How does the immune system work?

Every cell has its own tag which helps our body to know if the cell is familiar or not. When a foreign cell (antigen) enters the body it is quickly detected and several types of cells then work together to identify whether it is a threat to the body. If it is then antibodies (specialised proteins) are produced. They lock onto these antigens and then work with other cells to destroy them. If the body isn’t able to destroy the antigens straight away then the germs will multiply - causing you to feel ill. But, as your body destroys more of the germs you start to feel better.

Once produced these antibodies stay in a person’s body for life, meaning if those germs enter you body again you will be able to quickly identify them and destroy this. This is why for some diseases, such as chickenpox, you can’t normally get ill from it twice. This is also how immunisations work. They introduce the body to an antigen, without making the body sick, so that it will make antibodies and be protected from a future attack from that disease. This is known as immunity.

Antibodies also have a couple of other roles. They can neutralize toxins produced by different organisms and they also activate proteins which assist in killing bacteria, viruses or infected cells.

Immune system problems

There are four types of immune system disorders:

  1. Immunodeficiency disorders - these disrupt your body’s ability to defend itself against antigens
  2. Autoimmune disorders - when the body’s immune system attacks and destroys healthy body tissue. IBD is an autoimmune disorder
  3. Allergic disorders - a hypersensitive reaction to an allergen that has no effect on most people e.g. pollen, nuts
  4. Cancers of the immune system - such as lymphoma

What are autoimmune disorders?

IBD is an autoimmune disorder. An autoimmune disorder is where the immune system mistakenly attacks the body’s healthy cells as though they are germs. This then triggers inflammation in the body. In IBD the body is attacking parts of the digestive system. Doctors don’t yet know why this happens.

Why is inflammation triggered?

Inflammation is part of the body’s immune response. When tissue is damaged it helps to release chemicals into the blood which assist white blood cells to get to where they are needed. They also attract the specialist white blood cells, known as neutrophils and monocytes, to the area. These cells then help protect against an infection developing.

The immune system and IBD

It is believed IBD is the result of an abnormal response by your body’s immune system. The immune system mistakes food, bacteria and other materials as invading substances. White blood cells flood the area and inflammation takes place.

This inflammation is systemic (or body wide) creating the other aches and pains and fatigue associated with IBD.

Researchers also believe that once the immune system is ‘on’ it doesn’t know how to turn itself ‘off’ at the right time. This means the inflammation remains, causing damage and the symptoms of IBD. Many medical therapies - such as immunomodulators and biologics - for IBD focus on helping patients to regulate their immune system better.

The main medications for IBD which target the immune system are:

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