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 additional 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 to the early stages of an infection.
In most cases, a person who is experiencing a reaction has an underlying medical disorder which is causing the elevated white blood cell count. While the reaction itself is not dangerous, the medical disorder which has caused the reaction is often potentially harmful. There are many possible causes of leukemoid reactions.
Certain types of chronic infection often cause increased levels of white cells. These include diseases such as mononucleosis, malaria, and tuberculosis, which can persist in an inactive state for months or even years. The persistence of the infectious bacteria causes white blood cell levels to become chronically elevated.
Certain types of medications may also result in a leukemoid reaction. Treatment with a drug called Dapsone may increase white blood cell levels; this drug is used to treat infections such as tuberculosis, leprosy, and malaria, as well as certain autoimmune conditions. A medication called Tretinoin, used for certain dermatological disorders, may also cause these reactions.
A traumatic injury which causes a hemorrhage may also lead to an elevated white blood cell count. When this occurs, white blood cell levels tend to become elevated for several days, or longer, following the injury. In particular, spleen hemorrhage, or surgical spleen removal, can result in an increase in white blood cells. Leukemoid reactions are common following spleen injury or removal because the spleen is an immune organ in which large quantities of white blood cells are located.
Certain types of cancers, 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 elevated cell count caused by this condition. Under the microscope, however, the blood sample of a person with a leukemoid reaction often closely resembles a blood sample from someone with leukemia. This means it is sometimes difficult to distinguish between the two conditions.