There are many types of white blood cells all with a specific role to play in the defence of our body. One type of white blood cell known as lymphocyte is a true warrior when it comes to quick response to specific invaders of our bodies. Once the body has been invaded by a pathogen(bacteria, virus) the type of lymphocyte able to react to that particular pathogen is activated. The activation occurs when white blood cells known as phagocytes engulf the pathogen and present it to the lymphocyte.

Lymphocytes usually stay within lymph nodes. Once activated the lymphocyte grows in size and divides, giving rise to many clones. The lymphocytes then leave the lymph nodes travel to all parts of the body where they hunt and destroy the specific pathogen that was identified.