Wikijunior:Human Body/Immune System

The immune system is not a single object like many of the other organs discussed in this book but a lot of cells spread all through the body to protect us from many kinds of illnesses.

The immune system isn't something you can see without a microscope, because it is made of a lot of solitary cells which wander and patrol the body and cooperate to eliminate bacteria viruses and cells of the body that have become sick. This is why the immune system is called system instead of an organ. The only structures that can sometimes be seen and felt are lymph nodes and the thymus. Lymph nodes are little round organs where immune cells gather to exchange information and reproduce if there is an infection in the body. The gathering of immune cells in the lymph node can make it swollen.

The immune system is composed of white blood cells, lymph nodes, the bone marrow and the thymus.

The immune system is the police force of the body. Almost every cell and space in the body is patrolled to find sickness. There are a few different kinds of cells that make up the immune system.

• Macrophages eat bacteria, viruses, abnormal cells and debris. They warn T-cells about viruses and bacteria.
• T-cells patrol cells of the body and make sure they are o.k. They get signals from macrophages. T-cells coordinate a lot of the immune response. They tell B-cell to produce antibodies and they can kill abnormal cells that are then absorbed by macrophages.
• B-cells produce antibodies. Antibodies are small molecules that attach to certain structures at other cells like bacteria. By clotting the surface of bacteria they become inactive and can be devoured by macrophages. Each B-cell produces a specialized antibody that targets a specific bacteria or virus. Once a B-cell became active it stays on alert. This is why you suffer from a children's disease only once and why you are immune to a common cold in winter after you went through it.

There three types of organs that are part of the immune system:

• The bone marrow is where all blood cells of the immune system are produced.
• The thymus is the training station of the T-cells. Here, T-cells learn to distinguish between structures that belong to the body and foreign cell structures of bacteria and viruses.
• Lymph nodes are communication centers of the immune cells. Here macrophages inform T-cells which in turn trigger B-cells to produce antibodies. There multiple lymph nodes in the human body such that close to every part of the body there is such a communication center.

Immune cells have the privilege to be able to visit almost any part of the body. Like the police they have special rights. Immune cells can travel in blood vessels but if they come close to a place where an infection occurs they can leave a capillary to travel right to the place of action and take part in the scuffle.

T-cells patrol normal human cells. They do so by snuggling a cell and telling it to show part of what is inside. You can think of this as a passport control. The T-cell has to decide whether the passport is correct or not, thereby deciding whether the cell is normal or not. Apart from this, the immune system mostly deals with intruders from the outside. They have to be identified and after this the immune system is quite rigorous in eliminating those intruders. Records are kept by some T-cells to make sure next time the same intruder is expelled even more rapidly, sometimes without us noticing.

The immune system is very delicate. If you are sad or unhappy, the immune system may become weaker. Get a lot of sleep every night and don't do things that make you scared or unhappy. Spend time with friends and in friendly, safe places.