204 A MANUAL OF BACTEEIOLOGY 



kinds of cytases, one " macrocytase," obtainable from 

 tissues, such as the spleen and lymph- glands, rich in 

 macrophages, which acts specially on elements of animal 

 origin, the other " microcytase," derived from the micro- 

 phages, and which acts principally on micro-organisms. 

 He considers the alexic action to be of the nature of a 

 digestive process (but this is doubtful), and as regards the 

 complex nature of a cytolytic serum, which contains ambo- 

 ceptor and complement, believes that the amboceptor is 

 formed within the macrophages in intra-cellular digestion, 

 and that a portion of it escapes from them into the serum. 

 All the facts point to the leucocytes and leucocytic tissues 

 being the great defensive mechanisms against parasitic 

 invasion, either by the production of alexins, or of bacterio- 

 lysins, or by phagocytosis, or probably by a combination 

 of these (the " cellulo-humoral " hypothesis of immunity). 

 It is probable that the greater part of phagocytosis takes 

 place in the spleen. This organ acts as a sort of filter, 

 and phagocytosis may be active in it when none can be 

 discerned in the blood. Phagocytosis is also active in the 

 bone-marrow. 



Experiments by Tizzoni and Cattani seemed to show 

 that rabbits could not be rendered refractory to tetanus 

 by injection of tetanus antitoxin after extirpation of the 

 spleen ; and although Benario and other observers have 

 not confirmed this, the manner in which the spleen is 

 attacked in such diseases as tuberculosis, plague, etc., 

 points to this conclusion. The discordant results obtained 

 after splenectomy may be due to the rapid regeneration 

 of spleen tissue, and to other structures, such as the 

 hsemolymph glands, taking on its functions after ablation. 



Although small amounts of antitoxin may occasionally 

 be met with in the normal animal (e.g. diphtheria anti- 

 toxin in man and in the horse, see pp. 153 and 274), this 

 substance plays little or no part in natural immunity 



