White blood cells acts as soldier and scavengers in the body and are mobilised to fight against infection or remove waste debris. Different white blood cells play different roles; neutrophils against bacteria infection, lymphocytes against viral infection, monocytes act as scavengers and eisonophils against parasitic infections and allergic conditions. White blood cells will change in numbers and types of cells in response to the infective agent. Sometimes abnormal cells called atypical mononuclear cells (AMC) are also present. When leukemia occurs, the number of white blood cells will be markedly increased and immature cells called blasts will replace the normal cells.