During the past few years much accurate and careful work has been done to establish the exact influence of altitude on the blood. The accumulated evidence would seem to have settled the fact that there is a definite increase in the number of erythrocytes and in the content of hemoglobin. At a moderate altitude of about 6,000 feet this increase is at least 10 per cent. in both instances. No increase in the total number of leukocytes has been found. Little attention was paid to the differential blood-count at various altitudes until recent years.
In April 1909, Webb and Williams1 first called attention to an absolute increase in the lymphocyte or mononuclear element of the blood as an effect of altitude. In this and the previous year evidence regarding the defensive behavior of the lymphocytes in tuberculosis had been brought forward, notably by Bartel,2 Bergel3