What is immunity?

Our natural world is jaw-droppingly complex and beautiful. From the flora of our rainforests and woodlands, to the fauna of our countrysides and deserts, just thinking of all that complexity fills us with wonder.

However, much of the natural world we love wouldn’t exist, were it not for a multitude of tiny microorganisms. From bacteria and fungi to protozoa and viruses, these microbes may be too small to see with the naked eye, but their impact on our world is enormous.

While many of them are beneficial to us humans, there are also many types of microorganism that can actually do us harm. These are called pathogens, and they cause disease in human beings.

As a defence against these tiny invaders, over the millennia we’ve developed the immune system. Acting as both the castle wall and the defending army within, the immune system detects and tries to eliminate organisms that would do us harm.

But how exactly does it accomplish this? To answer that, let’s take a closer look at which parts of the body make up the immune system.

What is the immune system?

The immune system is a group of organs, cells and other proteins which protects u against damage from pathogens. A pathogen is a disease-causing germ such as fungi, bacteria, viruses and parasites.

To be able to cause disease, pathogens must first get past our front-line defenders. These include our skin, as well as mucous membranes lining entryways into the body (such as the nose and the mouth). Sneezes and coughs rid us of some pathogens by ejecting them before they ever get into our system. Meanwhile, pathogens which enter the body on our food are tackled by our stomach acid, preventing them from getting into the gut where they could cause damage.

However, some of the most stubborn pathogens do make it into our bodies. The immune system also works to neutralise these organisms, preventing them from doing us harm.

‘Me, or not me? That is the question.’

While the immune system is often concerned with identifying and destroying troublesome invaders, it can also help address threats from the inside,

One example of this is cancerous cells. These abnormal cells are potentially very damaging, so the immune system also plays a role in clearing them away.(1) It also cleans up any dead or defective cells in our system.

To be able to do this, it is essential that the immune system can recognise which healthy cells belong to you, and which are invaders or dangerously abnormal cells. Most of the time, your immune system is exceptionally good at this. However, sometimes it makes mistakes. It is not always able to detect cancerous cells, for example. or can work against healthy cells in the body.

When your immune system attacks your own healthy cells, this is called an autoimmune response. People who suffer from an autoimmune disorder face a lot of very challenging health problems.

The innate and adaptive immune systems

The immune system itself is comprised of organs, cells, tissues, and proteins. They work together in intricate ways.

However, scientists often talk about two different types of immunity.

The first is the innate immune system, or ‘non-specific’ immune system. When it comes to pathogens, the innate immune system is not at all choosy. It battles any new invader that looks suspicious, primarily using specialised white blood cells. These are called natural killer cells – which, as their name suggests, destroy invaders – and phagocytes, which engulf and break down pathogens.

On the other hand, the adaptive immune system targets germs that the body is already familiar with. It produces antibodies, which are tiny molecules which latch onto an ‘antigen’, a protrusion on the outside of a pathogen. (Interestingly, ‘antigen’ is actually short for ‘antibody generator’.(2))

Each set of antibodies is specific to just one threat, but once you have recovered from a disease a few antibodies stick around in your system. This means that, should the pathogen ever appear again, the body can initiate an effective immune response very quickly.

Immunity before life starts

Any parent will know that kids are prone to picking up bugs. As it turns out, there’s a reason for this.

The adaptive immune system learns over time. So, while an adult may have come into contact with lots of different pathogens, and have raised an immune response against them already, children will not have been exposed to the same degree.

Each time they encounter a new bug, their bodies have to raise an immune response to fight it off. Later, as they acquire more antibodies, they will seem to pick up fewer bugs. This is because the body can fight the germs off before the child shows any symptoms.

But if it takes a while to acquire immunity, wouldn’t that leave young babies very vulnerable to any pathogens that come along?

Fortunately, mothers can pass on temporary immunity to newborn babies to see them through their most vulnerable period. This happens when the mother passes on her antibodies through breast milk or while the baby is still in the womb. Eventually, the baby will start to build up his or her own immune system through exposure to pathogens in the outside world.

Soldiers and organs in the immune system…

On the frontline of the very efficient army that we call the immune system are small white blood cells, or leukocytes.

All leukocytes are part of both the adaptive and innate immune systems, and they are made in your bone marrow. Leukocytes include phagocytes (the ‘engulfing’ cells we mentioned above) which are mainly involved in the innate immune system.

The other main type of white blood cells are called lymphocytes. These play a main part in the adaptive immune system.

Lymphocytes assist in the identification of pathogens which have previously entered the body. There are two types, called B lymphocytes and T lymphocytes. Each of these have distinct functions.

Speaking very broadly, B lymphocytes tag foreign cells for destruction by flagging them with antibodies, while T lymphocytes do the destroying. They also activate and modulate other cells in the immune system.

White blood cells later go to live in the lymphoid organs. Lymphoid organs are the tonsils and adenoids; lymph nodes; lymphatic vessels; thymus; spleen; Peyer’s patches; and appendix as well as the bone marrow. They end up distributed all around the body.

Lymphocytes travel to the various organs through the lymphatic vessels which run alongside the arteries and blood vessels. A strong partnership exists between these systems of vessels, enabling them to pass cells and fluids easily back and forth. The lymphatic system also produces lymph, a clear fluid which cleanses human tissue.

What about inflammation?

The immune system is closely linked with inflammation, but what is inflammation and how does it come about?

Inside the body, inflammation is linked to the ‘complement system’. This is another vital part of your immune system made up fo 25 special proteins, so named because they ‘complement’ the action of antibodies. The complement system rids the body of pathogens which have become covered in antibodies.

However, in doing this the proteins enlarge your blood vessels, which can cause the symptoms of inflammation. These haven’t changed since the Roman medical writer Aulus Cornelius Celsus wrote about them in 1 AD, describing the signs as redness (rubor in Latin); heat (calor); swelling (tumor) and pain (dolor).

Contemporary wisdom on the immune system

There is still much about the workings of the immune system that we don’t understand. Modern scientific research continues to delve into our anatomy to answer these vital questions.

Scientists have made some fascinating discoveries in recent years. We’d like to share a few of the most interesting leads with you, here.

Mapping the immune system

Researchers from several institutions and universities pooled together to create a first cell ‘atlas’ of the thymus gland. The thymus is one of the main lymphoid organs.

The atlas enables scientists to track how a healthy thymus develops in humans. The thymus is most active in childhood; by the time you reach the age of 35, its job is largely over. So, if we don’t need the thymus our entire lives, why is it so important to map it?

The thymus produces the previously mentioned T white blood cells, vital in fighting bacteria and infection and also in killing cancer cells. Understanding the lifespan of the thymus also means that we gain insight into how we can fight cancer.

In the words of Professor Muzlifah Haniffa of the Wellcome Sanger Institute, a participant in the project: “This is really exciting as in the future, this atlas could be used as a reference map to engineer T cells outside the body with exactly the right properties to attack and kill a specific cancer — creating tailored treatments for tumours.”(3)

The thymus atlas is fascinating and exciting research which could have huge implications for bolstering the immune systems of vulnerable patients.

How the body triggers inflammation

As we mentioned earlier, the immune system triggers inflammation as a healthy, normal response to infection. However, sometimes inflammation happens when there is no infection. This type of inflammation can escalate quickly and jeopardise healthy tissue.

This is precisely what happens in inflammatory diseases such as Alzheimers’ disease and diabetes.

Can we understand more about how the immune system triggers inflammation and how it keeps inflammation in check? That is key to more effective treatment of these and other inflammatory diseases.

Researchers at the Institute of Cancer Research in London have discovered how to ‘turn off’ special cell structures called inflammasomes. Among other things, inflammasomes trigger pyrotopsis, a process where cells expand, explode, and then die. This in turn alerts other cells to proceed with inflammation.

Researchers now know how to enhance a group of proteins called SUMO proteins, which help keep inflammasomes under control. This could be an important step towards better treatment of inflammatory diseases.(4)

Stress and immunity

We have long known that there is a strong link between stress and physical health. But we still have a lot to learn about how the immune system interprets stress.

Research done by Steven Maier at the University of Colorado fills in some of the blanks. Maier found that the immune system reacts to stress in the same ways that it reacts to infection. In other words: stress is an infection!

When our bodies encounter an infection or injury, the brain releases a nonspecific immune response. This response both harnesses our resources and builds up energy to fight the threat. It includes both physical and behavioural responses, none of which are particularly pleasant, unfo