The role of the immune system is to deal with any infectious microorganisms that enter the body. An immune response always involves white blood cells. There are several different types and they all have different jobs to do.
Anything that gets into the body should be picked by straight away by a certain type of white blood cell. These white blood cells are able to detect things that are 'foreign' to the body, e.g. microorganisms. They then engulf the microbes and digest them. These white blood cells are non-specific - they attack anything that's not meant to be there.