The word leucocyte is formed from the Greek words leucos meaning colorless and kytos meaning cell. Colorless irregular nucleated cells of blood are called white blood cells. Lymphocytes and phagocytes are vigilant guards and fight against pathogens. Kapffer cells in the liver are a type of white blood cell. If the number of white blood cells is more than normal, it is called leukocytosis and if it is less, it is called leukopenia. When its number decreases, immunity decreases. If the number of white blood cells increases, cancer is called leukemia.