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Acute lymphoblastic Leukaemia (ALL), also known as acute lymphocytic leukemia, is a cancer that affects the lymphoid blood cell lineage. It is the most common Leukaemia in children, and it accounts for 10-20% of acute leukemias in adults. The prognosis for both adult and especially childhood ALL has improved substantially since the 1970s. The 5- year survival is approximately 95% in children. In adults, the 5-year survival varies between 25% and 75%, with more favorable results in younger than in older patients.

Lymphocytes include: 1) B cells, that produce antibodies to help fight infection; 2) T cells, that help B cells to fight infection; 3) Natural Killer (NK) cells that attack cancer cells and viruses.ALL is caused by lymphoid blasts, or lymphoblasts, and may be classified as B cell precursor ALL, mature B-cell ALL, or T ALL. NK cell Leukaemia is very rare.

B-cell ALL commonly presents with fever, fatigue, bone or joint pain, bleeding or anorexia (signs of bone marrow infiltration). T-cell ALL most commonly presents with anterior mediastinal mass, which may cause variable symptoms, from dry cough or shortness of breath to a medical emergency called superior vena cava syndrome (obstruction of blood flow through the superior vena cava). Generalized lymphadenopathy (enlarged lymph nodes) and hepatosplenomegaly (enlarged liver and spleen) are also frequent in T-cell ALL.

Acute Lymphoblastic Leukaemia is diagnosed when abnormal lymphoblasts are present in bone marrow or in circulation. Although there is no consensus regarding a minimal proportion of lymphoblasts in bone marrow, the diagnosis of ALL should be avoided when there are < 20 percent of lymphoblasts. Under normal circumstances, blasts don't make up more than 5% of bone marrow cells.

The American Cancer Society’s estimates for acute lymphocytic Leukaemia (ALL) in the United States for 2019 (including both children and adults) are: