IMMUNITY. 87 



found that certain leucocytes are active in the destruction of 

 various bacteria, yeast and tissue debris. These investi,q;ators 

 designated those leucocytes active in the destruction of bacteria, 

 phag(jcytes. Phagocytosis is a state or condition characterized 

 by tlie dc\-elopment of i.)hag'ocytes and the display (jf their 

 special function. The supporters of this theorv hold that the 

 cells, which arc active in the production <if leucocytes transmit 

 the property of phagocytosis to their progeny and thus inimun- 

 ity is perpetuated after it has been ac(|uired. That phagocytes dn 

 incorporate bacteria and other foreign substances is not denied 

 but it has not been demonstrated whether phaogocytosis is the 

 cause or the result of immunity. 



This theor}- does not explain immunitv from such diseases as 

 tick fever. The microzoon of tick fever inhal)its and usually 

 destr(i_\-s the red corpuscles. The leucocytes are proljabh' mil 

 affected hv them. In fact, the presence of the I'iroplasma Ijigeni- 

 inum in leucoCA'tes has not been noted. More recentlv Wright 

 and Douglas have demonstrated that certain suljstances in the 

 blood senun are necessar\' to prepare bacteria for phagrjcytic 

 actirm. These substances ha\'e Ijeen designated opsonins." 

 L')psonins are chemic su1)stances in blood serum that render 

 bacteria subject to the action of phagricytes. ( Jpsonins resem- 

 ble the amboceptors of Ehrlich in action, but they arc not iden- 

 tical with them. The action of opsonins is e\-idenced in |)neu- 

 monia, p\"ogenic infections, tuberculosis and i)robably in other 

 diseased crmditions. The opsonic index indicates the relative 

 power of resistance due to phagocytic action in an animal body. 



4. Humoral thcorx. — .\fter the phagocytic theory had ben 

 found insuiffcient, immunity was explained from a chemic view 

 point. The supporters of this theory, among whom P>uchner 

 was active, demonstrated the bactericidal action of blood serum 

 and Ivmph obtained from immune animals. Their demonstra- 

 tions ' established the fact that immunity is due to a chemic 

 substance, possiblv an enzyme. But the origin and specific ac- 

 tion of the chemicals in the production of immunity was not 

 determined. The bacteriolytic substance of the body fluid called 

 complement, was found to be destroyed by a temperature of 

 55°C. 



5. EhrUch's Lateral Chain Theory. — Ehrlich mai-ntains that 

 every livino- cell contains an active central body and an indefin- 

 ite number^ of marginal chemic groups or lateral chains desig- 

 nated receptors. These receptors combine with and extend the 

 nutriment to the central portion of the cell. The receptors are 



