Leukemoid reaction has been explored in Question 9.3 from the first  paper of 2008. This is a hyperproliferation of leukocytes, typically neutrophils. The numbers can rise to ridiculous levels - 30,000-50,000 are not uncommon. There is a good article on this topic, which is unfortunately not available without a subscription. Another similar article is less frequently referenced, but is infinitely more useful to the impoverished masses, as it has been made free-to-view by jpathology.com. In this article, there is a table of the "common and usual causes of leukemoid reaction".

I reproduce a part of it here, with no permission whatsoever:

Causes of Leukemoid Reaction
  • Infection
    • Pneumonia
    • Tuberculosis
    • Pertussis
    • AIDS/HIV
  • Drugs
    • Corticosteroids
    • G-CSF
    • All trans-retinoic acid (ATRA)
  • Increased neutrophil release
    • Stress
    • Trauma
    • Exercise
    • Sepsis
  • Inflammatory conditions
    • Empyema
    • Organ necrosis, eg. ischaemic hepatitis
    • DKA or HONK
  • Malignancy
    • Myeloproliferative disorders
    • Myeloid leukaemia
    • Lymphoma
    • Solid tumours, eg. lung
  • Decreased neutrophil clearance
    • Splenectomy

In a good-going leukemoid reaction, the granulocyte progenitor cells are too rushed to produce mature neutrophils. Typically, early neutrophil precursors swarm uselessly in the peripheral blood (eg, myelocytes, metamyelocytes) and the more severe the reaction, the earlier and less mature the circulating forms.

Apparently, about n half of these are associated with infection. According to a case series by Potasman et al (2013) the next most common cause is the ischaemia of a large organ, be it liver, transplanted kidney or gangrenous bowel. Together, ischaemia and sepsis accounted for about 80% of leukaemoid reactions; drugs obstetric procedures and paraneoplastic phenomena contributed the rest. In the majority of cases, the leukaemic "spike" in WCC lasted only one day.

So, is it leukemoid reaction or is it in fact a neutrophilic leukemia? How does one discriminate between the two? The most important aetiology to rule out is a myeloid leukaemia. A bone marrow biopsy soon reveals which is which.

References

Bain, Barbara J. "Diagnosis from the blood smear." New England Journal of Medicine 353.5 (2005): 498-507.

Sakka, Vissaria, et al. "An update on the etiology and diagnostic evaluation of a leukemoid reaction." European journal of internal medicine 17.6 (2006): 394-398.

ul Haque, Anwar, and Noor ul Aan. "Leukemoid Reaction: Unusual Causes." International Journal of Pathology 8.1 (2010): 39-40.

Weiner, W., and Elizabeth Topley. "Döhle bodies in the leucocytes of patients with burns." Journal of clinical pathology 8.4 (1955): 324-328.

Potasman, Israel, and Moti Grupper. "Leukemoid reaction: spectrum and prognosis of 173 adult patients." Clinical Infectious Diseases 57.11 (2013): e177-e181.