In estimating the clinical importance of a leucocytosis, it is not sufficient merely to count the aggregate number of leucocytes present.  A differential count must be made to determine which variety of cells is in excess.  In the majority of surgical affections it is chiefly the granular polymorpho-nuclear neutrophile leucocytes that are in excess (ordinary leucocytosis).  In some cases, and particularly in parasitic diseases such as trichiniasis and hydatid disease, the eosinophile leucocytes also show a proportionate increase (eosinophilia).  The term lymphocytosis is applied when there is an increase in the number of circulating lymphocytes, as occurs, for example, in lymphatic leucaemia, and in certain cases of syphilis.

Leucocytosis is met with in nearly all acute infective diseases, and in acute pyogenic inflammatory affections, particularly in those attended with suppuration.  In exceptionally acute septic conditions the extreme virulence of the toxins may prevent the leucocytes reacting, and leucocytosis may be absent.  The absence of leucocytosis in a disease in which it is usually present is therefore to be looked upon as a grave omen, particularly when the general symptoms are severe.  In some cases of malignant disease the number of leucocytes is increased to 15,000 or 20,000.  A few hours after a severe haemorrhage also there is usually a leucocytosis of from 15,000 to 30,000, which lasts for three or four days (Lyon).  In cases of haemorrhage the leucocytosis is increased by infusion of fluids into the circulation.  After all operations there is at least a transient leucocytosis (post-operative leucocytosis) (F.  I. Dawson).

The leucocytosis begins soon after the infection manifests itself—­for example, by shivering, rigor, or rise of temperature.  The number of leucocytes rises somewhat rapidly, increases while the condition is progressing, and remains high during the febrile period, but there is no constant correspondence between the number of leucocytes and the height of the temperature.  The arrest of the inflammation and its resolution are accompanied by a fall in the number of leucocytes, while the occurrence of suppuration is attended with a further increase in their number.

In interpreting the “blood count,” it is to be kept in mind that a physiological leucocytosis occurs within three or four hours of taking a meal, especially one rich in proteins, from 1500 to 2000 being added to the normal number.  In this digestion leucocytosis the increase is chiefly in the polynuclear neutrophile leucocytes.  Immediately before and after delivery, particularly in primiparae, there is usually a moderate degree of leucocytosis.  If the labour is normal and the puerperium uncomplicated, the number of leucocytes regains the normal in about a week.  Lactation has no appreciable effect on the number of leucocytes.  In new-born infants the leucocyte count is abnormally high, ranging from 15,000 to 20,000.  In children under one year of age, the normal average is from 10,000 to 20,000.