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What is Leukemoid reaction and explain in detail?
The term leukemoid reaction describes an increased
white blood cell count (> 50,000 cells/μL), which is a physiological response to stress or infection (as opposed to a primary blood malignancy, such as leukemia). It often describes the presence of immature cells such as myeloblasts or red blood cells with nuclei in the peripheral blood.
It may be lymphoid or myeloid.

Causes
As noted above, a leukemoid reaction is typically a response to an underlying medical issue. Causes of leukemoid reactions include:
Severe hemorrhage (retroperitoneal hemorrhage)
Drugs
Use of sulfa drugs
Use of dapsone
Use of glucocorticoids
Use of G-CSF or related growth factors
All-trans retinoic acid (ATRA)
Ethylene glycol intoxication
Infections
Clostridium difficile
Tuberculosis
Pertussis
Infectious mononucleosis (lymphocyte predominant)
Visceral larva migrans (eosinophil predominant)
Asplenia
Diabetic ketoacidosis
Organ necrosis
Hepatic necrosis
Ischemic colitis
As a feature of trisomy 21 in infancy (incidence of ~10%)
As a paraneoplastic phenomenon (rare)

Diagnosis
Conventionally, a leukocytosis exceeding 50,000 WBC/mm3 with a significant increase in early neutrophil precursors is referred to as a leukemoid reaction