How does the body fight invaders?
The immune system has two approaches to attacking invaders:

1. Ingestion by certain migrating cells, such as macrophages and a type of white cell known as neutrophils.
These cells are like an amoeba.  They move around the body and actually "eat" germs. Once inside the white cell, enzymes break down and destroy the invader. Cancer cells, which the immune system recognizes as foreign, are also combated in this way.

2. Binding and inactivation by protective molecules, called antibodies.
These either circulate in blood and lymph vessels or are attached to membranes of cell surfaces, as described previously. When they bind to foreign molecules, they make it easier for the invaders to be broken down. In the case of viruses, the binding can also stop the division of the virus.

Immunity to certain diseases and infections can be developed in two ways:

1. Natural Immunity
This is the kind that you are born with. It is genetic. There are differences  in immunity among species. For example, humans don't catch dog distemper. Dogs don't catch human colds.

2. Acquired Immunity
There are two ways to acquire immunity:

1. Active response to infection or exposure to certain substances. This means that you actually come down with the illness, infection, or virus, but develop the defenses to fight it off and avoid it in the future. The active response takes several days to build up, but the protection may last several years or more. 

2. Passive transfer of antibodies from certain vaccines or from the milk of nursing mothers.  The passive protection from certain vaccines or mother's milk provides immediate protection, but the effects may only last a few days or weeks.

"Sneaky Pete" Organisms: some microbes hide from the body's defense system by inserting themselves into the cells. The parasite that causes malaria is a good example; it hides inside red blood cells.

Let's focus on the white blood cells for a moment, since they are at the center of the immune system.

For more about infectious disease, check the "Kids Health" web site; click here.

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