Leukaemia is a cancer of the white blood cells. Acute lymphoblastic leukaemia is a cancer of immature lymphocytes, called lymphoblasts (sometimes referred to as blast cells). Normally, white blood cells repair and reproduce themselves in an orderly and controlled manner but in leukaemia the process gets out of control and the cells continue to divide, but do not mature.
These immature cells fill up the bone marrow and prevent it from making blood cells properly. As the leukaemia cells do not mature, they cannot do the work of normal white blood cells, leading to an increased risk of infection. And because the bone marrow is overcrowded with immature white cells it cannot make the right numbers and quality of red cells and platelets. As a result, symptoms such as anaemia and bruising also occur.
Although acute lymphoblastic leukaemia is more common in children, adults do also get the disease.