METABOLISM IN VERTEBRATES 



from prothrombin, and, in turn, fibrin is formed from fibrinogen; a clot 

 occurs. When drawn blood clots in a bowl and is allowed to stand, the clot 

 contracts, and a pale yellowish fluid is squeezed out. This fluid is what re- 

 mains of the original plasma. It is called blood serum and contains, among 

 other things, the substances which immunize against certain diseases. 



One very important function of the blood is the transport of oxygen in 

 combination with the hemoglobin of the red blood cells. Death results from 

 loss of one-third the blood in warm-blooded animals because oxygen is no 

 longer delivered to the cells of the body in adequate amounts. In medical 

 practice a blood transfusion may save a life after hemorrhage or in severe 

 anemia. When transfusion was first attempted, it was often found that the 

 patient died as soon as blood from another person was introduced into his 

 vessels. The reason was that the red cells of the donor blood clumped or 

 stuck together; they were agglutinated as a result of a reaction between 

 proteins of the plasma and proteins of the red cells. These clumps prevented 

 the free flow of blood in the capillaries and thus caused death. About 1900 

 it was discovered that the blood of any human being falls into one of four 

 types, depending on the presence in the red cells of either substance A or B. 

 of both A and B (AB), or of neither A nor B (O). About 46 per cent of the 

 human race have blood of type O; 42 per cent, type A; 9 per cent, type B; 

 and 3 per cent, type AB. 



The substances A and B are examples of what are called antigens. The 

 substances in the plasma which react with A and B to bring about agglutina- 

 tion are examples of antibodies. These are what are known as normal antigens 

 and antibodies. In an individual with type A blood the red cells contain 

 antigen A and the antibody for A does not occur in the plasma. Such in- 

 dividuals carry antibodies for antigen B. If blood of type B or type O is placed 

 in the vessels of a type A individual agglutination can occur because antibodies 

 for A are present in the plasma of type B and type O blood. In emergencies it 

 is sometimes possible to use type O blood to transfuse a type A or type B in- 

 dividual depending on the amount of blood required and the concentration of 

 antibodies A or B in its plasma. 



Proteins, such as those of bacteria, which are foreign to a certain kind of 

 animal and which invade its tissues are also known as antigens. The tissues 

 of the animal react to the presence of the foreign protein by producing anti- 

 bodies that combat the eflects of the antigen. Such antibodies are called 

 immune antibodies and become abundant in the blood plasma, where they 

 remain for varying lengths of time and provide immunity to reinvasion by the 

 foreign protein or antigen that conditioned their production. Thus, recovery 

 from a particular disease may result in permanent or transitory active acquired 

 immunity to that disease. 



The capacity of animals to synthesize substances protective against foreign 

 proteins can be utilized to protect human beings against diseases. If minute 

 quantities of the pathogenic material are introduced into the body, its tissues 

 will produce abundant antibodies, even though the amount of infective 



65 



