The _normal_ number of leucocytes per cubic millimetre varies in
different individuals, and in the same individual under different
conditions, from 5000 to 10,000: 7500 is a normal average, and anything
above 12,000 is considered abnormal. When leucocytosis is present, the
number may range from 12,000 to 30,000 or even higher; 40,000 is looked
upon as a high degree of leucocytosis. According to Ehrlich, the
following may be taken as the standard proportion of the various forms
of leucocytes in normal blood: polynuclear neutrophile leucocytes, 70 to
72 per cent.; lymphocytes, 22 to 25 per cent.; eosinophile cells, 2 to 4
per cent.; large mononuclear and transitional leucocytes, 2 to 4 per
cent.; mast-cells, 0.5 to 2 per cent.

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 leucæmia,
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