Leukemoid reactions are temporary increases in white blood cell count, often caused by underlying medical disorders such as chronic infections, medication, traumatic injury, or certain types of cancer. The reaction itself is not dangerous, but the underlying disorder may be harmful.
White blood cells are an important component of the immune system. When the immune system is activated, the number of white blood cells in the body increases; in some cases this increase is much greater than normal. This further increase in white blood cells is referred to as a leukemoid reaction.
Another name for a leukemoid reaction is transient myeloproliferative disorder. Transient refers to the fact that the elevated white blood cell count is temporary, while myeloproliferative refers to the fact that a large proportion of the additional white blood cells are immature myelocytes. This cell type differentiates into cells called neutrophils, basophils, and eosinophils, which are important in the immune response in the early stages of an infection.
In most cases, a person experiencing a reaction has an underlying medical disorder that is causing the white blood cell count to rise. While the reaction itself isn’t dangerous, the medical disorder that caused the reaction is often potentially harmful. There are many possible causes of leukemoid reactions.
Certain types of chronic infection often cause white blood cell levels to rise. These include diseases such as mononucleosis, malaria and tuberculosis, which can persist in an inactive state for months or even years. Persistence of infectious bacteria causes white blood cell levels to become chronically elevated.
Some types of medications can also cause a leukemoid reaction. Treatment with a drug called Dapsone can increase white blood cell levels; this drug is used to treat infections such as tuberculosis, leprosy, and malaria, as well as some autoimmune conditions. A drug called tretinoin, used for some skin disorders, can also cause these reactions.
A traumatic injury that causes a hemorrhage can also lead to a high white blood cell count. When this occurs, white blood cell levels tend to rise for several days, or longer, after the injury. In particular, bleeding from the spleen, or surgical removal of the spleen, can cause an increase in white blood cells. Leukemoid reactions are common following injury or removal of the spleen because the spleen is an immune organ that contains large numbers of white blood cells.
Certain types of cancer, including adenocarcinoma and Hodgkin’s disease, often lead to an increase in white blood cells. While leukemia also causes an increase in white blood cells, this is distinct from the high cell count caused by this condition. Under a microscope, however, the blood sample from a person with a leukemoid reaction often looks a lot like a blood sample from a person with leukemia. This means that it is sometimes difficult to distinguish between the two conditions.
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