The word leucocyte comes from the Greek, meaning "white cell". Leucocytes are also called white blood cells. They are a part of the immune system and are like "health police" for the body. They fight against bacteria, viruses, tumour cells, and toxins. This protects the body from many illnesses. Leucocytes use the bloodstream to travel through the body looking for these substances. Healthy adults have between 4000 and 10 000 leucocytes in every microlitre of blood, but this can increase enormously when someone is battling an infection.