What is the Immune System?
Your Immune System: Beating Disease and Keeping a Healthy Balance
Barriers to infection
The human body has to protect itself effectively against tiny bacteria and viruses (otherwise known as "germs") that can cause disease – especially since they would otherwise find it a particularly easy environment within which to settle and multiply due to the free 'food' and shelter available.
There are a number of ways to keep the body free from infection – one of the simplest, and most effective, is to create a physical barrier. For instance, the skin is very effective at blocking germs. In addition, certain substances produced by the body, such as saliva and tears, contain molecules that can neutralise germs, further enhancing the barrier effect.
In other parts of the body there are greater challenges, For instance, in the lungs, gases from the air need to be able to get into the body, so there is only a thin layer of tissue separating the air from the bloodstream. This could easily become a route for harmful bacteria and viruses to enter the body too. So, a sticky substance called 'mucus' coats the lining of the lungs and helps to capture germs and other foreign particles. Once captured, tiny moving hairs that also line the airways (known as cilia), transport the captured material away from the sensitive areas.
Similarly, in the gut, food needs to be absorbed easily into the bloodstream, so, again, the gut wall is relatively thin – so this is another area where security could be breached. As we shall see, the body has ways of dealing with these problems.
When security is breached…the immune system steps in
Sometimes, these methods aren't quite enough and germs are able to invade, so the body has to find other means to combat them.
Here it is useful to remember that, although in terms of size, the human body is relatively large, it is actually composed of millions upon millions of tiny cells knitted together. These cells tend to be specialised for a particular purpose and organised into 'tissues' – think of brain cells, skin cells and muscle cells, for example. Similarly, there are also specialised cells responsible for defending the body against invading germs – those that have managed to get through the barriers described above and enter tissues or the bloodstream, for example.
Some of these specialised cells are actually part of the tissue barriers we have just described, and are able to detect invaders as soon as they arrive. Other specialised cells spend their time moving around the body looking for signs of infection, or sit in particular areas of the body (such as in the lymph nodes or in the spleen) waiting for signs of infection to reach them via the blood circulation.
Single cells, being so small, are closer in size to germs and can often deal with them directly – and there are also other, even smaller, molecules that can also target bacteria and viruses.
These specialised molecules, cells, tissues and organs together form the immune system – and it is found throughout the body.
Certain bacteria and viruses ("germs") can cause disease if they get into ("infect") the body.
A relatively simple way for the body to defend itself is by using 'barriers' to block infection by germs. A good example of this is the skin. Other substances produced by the body, such as tears, can also neutralise germs.
The immune system is a network of cells, tissues and organs, found throughout the body that combats infectious disease and cancers. It is divided into 'innate' and 'adaptive' immune responses.