The immune system

Here is Selma on the train. She's happy, after a fun trip. Unfortunately, there is another passenger on the train carrying this. It is a measles virus. Measles is highly contagious.

Since Selma is sitting next to the virus carrier during the whole train ride, the virus reaches her body through the air. She becomes infected. But Selma's body has a system that does everything to prevent and destroy the intruders. This is Selma's immune system. And now a war breaks out between the intruding measles viruses and the immune system.

Selma becomes infected when she inhales the virus, and it reaches the lungs. The immune system gets started. These are the first soldiers on the scene. Their task is to kill off bacteria by eating them. They are scavenger cells.

They float around our blood vessels and are present in the whole body. But they have no power against viruses. Instead, the measles viruses invade the scavenger cells, reproduce in huge numbers, and the scavenger cells die. Now, the viruses spread in their millions, further and further into Selma's body. The war has been going on for about ten days, but it's not until now that Selma notices that she is infected.

And she becomes very ill. The temperature rises in the body. This makes it somewhat harder for intruding bacteria and viruses to reproduce. Selma has a fever. Fever is a part of the body's immune system.

And Selma coughs. And gets rashes. Coughing is the body's way of disposing of the bits of dead viruses and scavenger cells remaining in the lungs. Rashes are caused by masses of defense cells gathering around the viruses. Then the blood vessels expand, and on the skin this shows as red dots.

This is called an inflammation. What makes Selma uncomfortable when she is ill is not the viruses themselves, but the body's response to them: fever, coughing, and inflammation. Inflammation is found in many parts of Selma's body. If she is unlucky, the brain might become inflamed. If so, measles is life-threatening.

What does the immune system do now to protect Selma? Unfortunately it can't do much, yet. It hasn't got the right weapons. But it works hard to get them. Therefore the immune system sends out these: T-cells.

They carry broken parts of measles viruses, to present them in the lymph nodes. There, these B-cells can see what these viruses look like. Now, they can learn how to design and produce a weapon that is effective against these viruses in the body. The weapon is a certain type of protein, called antibodies. Lots of antibodies, that exactly match the particular virus that the T-cells brought, and presented.

And this time it's the measles virus that will be fought. When the antibodies are produced, Selma's immune system can begin the counterattack. Billions of antibodies are sent out into the body. They attach to the viruses, and to the cells that the viruses have entered. It's as if the viruses are marked with flags.

And then, an army of killer T-cells comes along. They destroy all the viruses and cells that are marked with the right antibody. T-cells and B-cells are different kinds of white blood cells. They are some of the soldiers in the immune system. Now it has been almost three weeks since the virus entered the body, and Selma's immune system begins to regain control.

It takes a few weeks before she feels fit and well, and it looks like she'll be back to normal again. But there's more to this! Now, the cells in Selma's immune system have learnt to recognise the measles viruses. Certain memory cells remain after the illness. Thanks to them, if Selma meets the same virus again, loads of antibodies can be produced, in a flash.

The cells know how to do it. Selma has become immune to measles. So, Selma recovered from the illness. And she will never get the measles again, even if she encounters the measles virus here and there. Thanks to the immune system.