Chemotaxis alone will not explain the continuance of leukocytosis for more than a short time. It is probable that substances which are positively chemotactic also stimulate the blood-producing organs to increased {158} formation of leukocytes; and in at least one form of leukocytosis such stimulation probably plays the chief part. In general, the response of the leukocytes to chemotaxis is a conservative process. It is the gathering of soldiers to destroy an invader. This is accomplished partly by means of phagocytosis--actual ingestion of the enemy--and partly by means of chemic substances which the leukocytes produce. In those diseases in which leukocytosis is the rule the degree of leukocytosis depends upon two factors: the severity of the infection and the resistance of the individual. A well-marked leukocytosis usually indicates good resistance. A mild degree means that the body is not reacting well, or else that the infection is too slight to call forth much resistance. Leukocytosis may be absent altogether when the infection is extremely mild, or when it is so severe as to overwhelm the organism before it can react. These facts are especially true of pneumonia, diphtheria, and abdominal inflammations, in which conditions the degree of leukocytosis is of considerable prognostic value. As will be seen later, there are several varieties of leukocytes in normal blood, and many chemotactic agents attract only one variety and either repel or do not influence the others. These varieties may be divided into two general classes: (_a_) Those having active independent ameboid motion. They are able to migrate readily from place to place and to ingest small bodies, as bacteria. From this latter property they derive their name of _phagocytes_. This group includes all varieties except the lymphocytes. The polymorphonuclear leukocytes are taken as the type of the group, because they are by far the most numerous. {159} (_b_) Those having very little or no independent motion--_non-phagocytic leukocytes_. Only the lymphocytes belong to this class. By this classification we may distinguish two types of leukocytosis, according to the type of cell chiefly affected: a phagocytic and a non-phagocytic type. 1. Phagocytic Leukocytosis.--Theoretically, there should be a subdivision of phagocytic leukocytosis for each variety of phagocyte, _e.g._, polymorphonuclear leukocytosis, eosinophilic leukocytosis, large mononuclear leukocytosis, etc. Practically, however, only one of these, polymorphonuclear leukocytosis, need be considered under the head of leukocytosis. Increase in number of the other phagocytes will be considered at another place. They are present in the blood in such small numbers normally that even a marked increase scarcely affects the total leukocyte count; and, besides, substances which attract them into the circulation frequently repel the polymorphonuclears, so that the total number of leukocytes may actually be decreased.