In the mouse, the total WBC count is significantly influenced by the site of blood collection and the time of day – variations up to 30 times can be within physiological limits. For example:
: 14,000/µl 6:00 am
: 21,150/µl 3:00 pm
: 7,000/µl 9:00pm
Males have higher counts after 4 weeks of age.
The mouse generally responds to stress with a myeloid response. Moreover, the differential count in mice is heavily weighted towards lymphocytes. For example the reference intervals in human differential blood counts are 45-62% neutrophils, 16-45% lymphocytes 20-36% mononuclear cells. In contrast, lymphocytes in the mouse blood, depending on the strain and gender, are between 45% to 90% of the white blood cells.
Most mouse strains are above 70% lymphocytes. Another difference to note is that the normal B lymphocyte population in mice is heavily weighted toward kappa light chain (2 to 1 in human but 9 to 1 in mouse). The combination of high percentage of lymphocytes and the kappa imbalance in light chain can mislead investigators to suspect a B-cell lymphocytic leukemia.
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