You learned previously that the human body is made up of systems. In this activity, you will look closely at another system: the immune system. 

The immune system works around the clock. When it fails, your body sends messages to the brain. The word immunity refers to your body's ability to fight infections. Your body can get an infection several ways:

• by germs are absorbed through the skin

• by germs enter with our food or drink

• by germs enter from the air we breathe

• by germs are trapped and absorbed through cuts in our skin

When germs enter our bodies, more white blood cells are needed to find the trouble makers and destroy them. Our body learns what the infection "looks like" and remembers it. Sometimes when the same germs try to cause trouble again, your body is able to destroy them before any harm is caused. This is why, for example, most people get chicken pox only once. We also receive vaccinations to help our body defend itself.

The image above shows the parts of the body that make up the immune system.

Your nervous system also plays a part in all of this. Your senses, nerves, and brain work together to get information to your brain to be processed; they help us deal with any changes around us.

Your immune system does its work largely unnoticed. One thing that causes us to notice our immune system is when it fails for some reason. We also notice it when it does something that has a side effect we can see or feel.

 Here are some examples:

• When you get a cut, bacteria and viruses enter your body through the break in the skin. Your immune system responds and eliminates the invaders while the skin heals itself and seals the puncture. Sometimes, the immune system misses something and the cut gets infected. It gets inflamed and will often fill with pus. Inflammation and pus are both side effects of the immune system doing its job.

• When you get a mosquito bite, you get a red, itchy bump. That too is a sign of your immune system at work.

• Every day, you inhale thousands of germs (bacteria and viruses) that are floating in the air. Your immune system deals with all of them without a problem. But occasionally a germ gets past the immune system and you catch a cold, get the flu, or worse. A cold or flu is a sign that your immune system failed to stop the germ. The fact that you get over the cold or flu is a sign that your immune system was able to eliminate the invader after learning about it. If your immune system did nothing, you would never get over a cold or anything else.

• Every day, you also eat hundreds of germs, and again, most of these die in the saliva or the acid of your stomach. Sometimes, however, one germ gets through and causes food poisoning. There is normally a very visible effect of this breach of the immune system: Vomiting and diarrhea are two of the most common symptoms.

• There are also all kinds of human ailments that are caused by the immune system working in unexpected or incorrect ways. For example, some people have allergies. Allergies are really just the immune system overreacting to certain things in the environment that other people don't react to at all.

• Some people have diabetes, which is caused by the immune system inappropriately attacking cells in the pancreas and destroying them. Some people have rheumatoid arthritis, which is caused by the immune system acting inappropriately in the joints.
  

• Finally, we sometimes see the immune system preventing us from doing things that would be otherwise beneficial. For example, organ transplants are much harder than they should be because the immune system often rejects the transplanted organ.