Leukemia is a cancer of the blood cells. In cancer, cells become abnormal and grow out of control. As the number of abnormal blood cells increase, the healthy blood cells are outnumbered. There are three main types of blood cells. Each has a distinct job:

  • White blood cells (WBC), also called lymphocytes, are most often involved in leukemia. Their main job is to help the immune system.
  • Red blood cells (RBC) carry oxygen throughout the body.
  • Platelets help the blood clot at injury sites.

Leukemia cells cannot do the job of normal blood cells. This causes many of the symptoms of leukemia. The disease starts in the bone marrow where blood cells are made. The most common types of leukemia are:

White Blood Cells
Immune system white blood cell
Leukemia cells outnumber healthy white blood cells and gather in spleen and lymph nodes.
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Cancer occurs when cells in the body divide without control or order. Normally, cells divide in a regulated manner. If cells keep dividing uncontrollably when new cells are not needed, a mass of tissue forms, called a growth or tumor. The term cancer refers to malignant growths. These growths can invade nearby tissues. Cancer that has invaded nearby tissues can then spread to other parts of the body.

It is not clear exactly what causes these problems in the cells, but it is probably a combination of genetics and environment.