How does the immune system work?


Dr. Jen Tan
@AVogelUK
Ask Dr. Jen Tan


17 March 2014

What is the immune system?

Our body’s immune system is extremely complex and many aspects of it have yet to be revealed by medical science. For instance, we still do not fully understand why some people are more prone to infections, such as the common cold or the flu, than others.

The immune system exists practically everywhere in the body. The most well-known cells of the immune system are the body’s white blood cells, often thought of as the body’s policemen. These move round the body in our blood stream. However, our blood vessels only represent the ‘motorways’ of our body – to be effective, immune system cells have the ability to leave the circulatory system and mingle amongst cells in our tissues and organs.

The primary function of the immune system is to protect us from infection and it does so by means of two types of action.

Non-specific immune action

This is referred to by scientists as innate or humoral immunity.

An infective organism entering the body causes injury to cells. These respond by releasing a number of chemicals known as eicosanoids and cytokines into the surrounding tissue.

These chemicals can be thought of as the 999 call asking for help. They trigger a number of effects, including bringing more blood into the infected area, signalling for more white blood cells to the area to help fight infection and if the infection is a severe one, raising the body’s temperature, giving rise to fever.

The result of all this is that the infective organism comes under attack. In addition, immune chemicals signal to a type of immune cells known as phagocytes, asking them to get to work – they have the unique ability to engulf or ‘eat up’ infective organisms and once inside the cells, enzymes are released, destroying the invader.

Specific immune system action

It is commonly known that we tend to pick up certain infections only once – examples include the childhood illnesses of measles and chicken pox. The reason for this lies with another part of our immune system known to scientists as adaptive or cell-mediated immunity.

The key to this type of immunity is the involvement of immune system cells (hence, cell-mediated) which have the ability to recognise infective organisms. This is possible because they can ‘look at’ and recognise different types of proteins present on the surface of viruses, bacteria or any other organism.

The first time the body is invaded by the chicken pox virus, the non-specific immune action described above is triggered.

At the same time, the body’s immune cells see what this virus looks like and remember it.

This memory is retained and the next time the same virus enters the body (and it could be years later), a stronger and more specific response is triggered because the body has seen the invader before, leading to a faster release of immune chemicals and destruction of the infective organism before it has the chance to do harm.

So why do some people get chicken pox twice?

For some people, the ability of the body’s immune system to remember what the virus looks like fades – this is similar to failing memory when one gets older. We do not fully understand why this occurs. It could be related to a general ‘weakening’ of the immune system because of illness, but in most, there is no obvious explanation.

So why do I get colds frequently? Will I not develop an immunity to these viruses?

There are over 200 types of viruses that cause colds. Many of these will have a number of sub-types and each will look different to your immune system. You may indeed have developed an immunity to many cold viruses, but each winter, many will come around which your body has never seen.

In addition to all this, viruses have a tendency to ‘mutate’ from season to season, changing the way they look.. So, to your body, it may appear to have blond hair one year, but when it appears the next year, it will have disguised itself and returned with black hair.

Influenza viruses, in particular, have this ability. The names H1N1 or H1N5 denote different variants of the same family of viruses. This is also the reason that flu vaccinations change from year to year as there is the need to protect you from the viruses most likely to be around.

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