The peculiar characteristics of the leukocytes have already been described at considerable length in the preceding chapter. It must suffice, then, here to say that during the _inflammatory_ attack the leukocytes are _increased in number_, _i. e._, there is a temporary leukocytosis which is the usual accompaniment of suppuration. For instance, this is regularly present in purulent, but not in catarrhal, forms of appendicitis. The recognition of this fact may be of great value in diagnosis. For instance, leukocytosis is rarely present in tuberculous disease unless suppuration complicates the case. It is met with in suppurative osteomyelitis and in all cases of pocketing of pus. Moreover, when leukocytosis is present coagulability of the blood is increased. Of the various leukocytes, it is the mononuclear and polynuclear forms (see Chapter II) which are endowed with the most pronounced activity and which play the principal role among the blood cells or phagocytes. That phagocytosis plays a most important part in the inflammatory process is a matter to be emphasized in more than one way and in more than one place. The account of the process already given should suffice for descriptive purposes; the importance of the act, however, should be made most prominent in considering inflammation and suppuration. That the phagocytic properties of these cells are limited will be remembered when we recall that in certain instances phagocytes, which are incapable of defence as against the mature bacterial organism, are yet capable of englobing the spores and preventing their development. Nevertheless, the activities of even the most lively phagocytes are capable of being influenced and repressed by extremes of heat and cold to which patients may be exposed, either locally or generally. CHEMOTAXIS AND OPSONINS. Having considered briefly the cells which take prominent part in the inflammatory process, and the escape along with them of the fluid portions of the blood, whether these coagulate or not, it is necessary before referring to specific factors to discuss that which induces the above cells to act in this way. That there is a peculiar, even a mysterious, attraction which brings specific irritant and phagocyte together has been for some time recognized, but it remained for Pfeffer to study it carefully and to give it the name by which it now passes, _i. e._, _chemotaxis_, while others have widened our knowledge of it, especially by a recognition of the _opsonins_ or material which “prepares food,” _i. e._, prepare microbes for ingestion by the phagocytes.