How does the immune system work?

First of all, a pathogen (for instance a virus or unfriendly bacteria) must get past the body's physical barriers (such as skin, mucous membranes, or nasal hairs) and its chemical barriers (such as stomach acid) to start attacking cells.

The cells under attack then release chemicals that attract immune cells called macrophages which start to deal with the pathogen. If the pathogen resists this, macrophages can call on T cells. T cells will secrete chemicals to help overcome the pathogen. They can also activate B cells that lock onto the pathogen. Once the pathogen is killed, those B cells will remember how to kill it. This is why adults often get fewer colds and flu than children – the have been exposed to more pathogens (viruses).