Definition

Immunity is the ability of an organism, through its immune system, to resist pathogens. It can work in two ways:

1. by preventing the entry of pathogens into the body in the first place (a barrier strategy), or
2. by attacking and destroying pathogens and other foreign material once they have entered (a seek-and-destroy strategy).

The immune system is the organised collection of organs, tissues, cells and chemicals that carries out these tasks.