BLOOD AND BLOOD DERIVATIVES — COHN 



433 



ease. In others the incidence of the disease in susceptibles is so small 

 that it would be necessary to immunize passively an entire population 

 in order to offer protection to the one in a thousand who might con- 

 tract the disease. The blood donors for such an undertaking might 

 not always be available, whereas blood donors conscious of the con- 

 tribution that they were making to the control of other diseases could 

 clearly always be counted on in a society that has contributed as has 

 ours to the blood donor program. 



The natural functions and therapeutic uses of blood. — Blood is a 

 complex tissue consisting of a fluid part, the plasma, in which cells of 

 various kinds are suspended. Most important among these are the 

 leukocytes or white cells, and the erythrocytes or red cells. 



The plasma also is complex. It is an aqueous solution of salts, carbo- 

 hydrates, and lipids, the latter, for the most part, present in combina- 

 tion with certain of the diverse protein molecules which are the chief 

 plasma constituents. 



The various components of the blood stream have such complex 

 interrelations, on the one hand with the tissues, on the other with the 

 external environment, that we may well stress the few systems where 

 an understanding of the mechanism is emerging. 



Transport of oxygen by the hemoglobin concentrated in the red 

 blood cells. — The red blood cells have as their prime function the 

 transport to the tissues of oxygen and the removal from the tissues of 

 carbon dioxide and other waste products. The oxygen is transported 

 in combination with the hemoglobin concentrated within the red cell. 

 Other proteins, notably carbonic anhydrase and a complex enzyme 

 system, represent its less copious protein components. 



When there is great blood loss from a wound, when a major opera- 

 tion is to be performed, or when the anemia which frequently occurs 

 in convalescence from wounds must be combatted, red blood cells 



