Leukemia is a type of cancer that originates in the blood-forming tissues, including the bone marrow. Leukemias are generally categorized into four main types:

  • Acute Lymphoblastic Leukemia (ALL): This type of leukemia is the most common type in young children but can also affect adults. It involves the overproduction of lymphoblasts, a type of immature white blood cell.
  • Acute Myeloid Leukemia (AML): AML affects both children and adults and is characterized by the rapid growth of abnormal myeloid cells that build up in the bone marrow and interfere with the production of normal blood cells.
  • Chronic Lymphocytic Leukemia (CLL): CLL typically affects adults over the age of 55. It's characterized by the slow growth and accumulation of mature, abnormal lymphocytes (a type of white blood cell) in the blood, bone marrow, and lymph tissues.
  • Chronic Myeloid Leukemia (CML): CML tends to occur in adults and involves an overproduction of abnormal granulocytes, a type of white blood cell.

In addition to the four primary types of leukemia there are several subtypes and less common types of leukemia:

  • B-cell Prolymphocytic Leukemia (B-PLL): A rare type of leukemia characterized by an excess of B-cell lymphocytes in the blood.
  • T-cell Prolymphocytic Leukemia (T-PLL): A rare and aggressive type of leukemia characterized by an excess of T-cell lymphocytes.
  • Hairy Cell Leukemia (HCL): A rare, slow-growing cancer of the blood where the bone marrow overproduces B cells.
  • Juvenile Myelomonocytic Leukemia (JMML): A rare and serious type of childhood leukemia characterized by the overproduction of myelomonocytic cells, which are immature white blood cells.
  • Large Granular Lymphocytic Leukemia (LGL Leukemia): A slow-growing leukemia where too many large granular lymphocytes, a type of white blood cell, are produced.
  • Adult T-cell Leukemia/Lymphoma (ATL): A rare and often aggressive type of T-cell leukemia caused by the human T-cell lymphotropic virus type 1 (HTLV-1).
  • Precursor Cell Lymphoblastic Leukemia-Lymphoma: A fast-growing type of leukemia that affects precursor cells that are meant to mature into healthy white blood cells.
  • Myelodysplastic Syndromes (MDS): Often considered a "pre-leukemia" condition, MDS is a group of disorders where the bone marrow does not produce enough healthy blood cells, and there's a risk of developing into acute myeloid leukemia.
  • Mixed Phenotype Acute Leukemia (MPAL): A rare type of leukemia where the cells have characteristics of both lymphoblastic and myeloid cells.

Each type of leukemia can have different prognoses and treatment options, so correct classification is critical. The 'acute' types are aggressive and progress rapidly without treatment, whereas the 'chronic' types tend to progress more slowly. The 'lymphoblastic' or 'lymphocytic' leukemias start in the cells that become lymphocytes, while 'myeloid' leukemias start in the cells that would normally go on to form red blood cells, platelets, and some types of white blood cells.