METABOLISM AND TRANSPORT IN ANIMALS 



527 



Thrombokinase + Prothrombin + Calcium = Thrombin 



Thrombin + Fibrinogen = Fibrin 

 Fibrin + Erythrocytes = Clot 



When the red blood corpuscles become 

 trapped in the fibrin network, a clot is 

 formed that prevents escape of blood from 

 the wound. Another substance circulating 

 in the blood, antithrombin, neutralizes the 

 effect of any excess thrombin. In certain in- 

 dividuals, where one or more of these fac- 

 tors may be lacking, the blood clots so 

 slowly that a minor wound results in severe 

 bleeding. This condition, known as hemo- 

 philia, is hereditary. 



Antibodies 



Foreign substances, usually proteins of 

 either plant or animal origin, when intro- 

 duced into the blood of an animal, will 

 cause the production of antagonistic sub- 

 stances collectively known as antibodies. 

 The antibodies which the body produces 

 when one is infected with a disease-causing 

 organism such as bacteria are a part of the 

 body's defense mechanism. They confer a 

 degree of immunity to further infection to 

 a given organism; this may be temporary 

 or permanent. Man has taken advantage of 

 this knowledge of immunity by injecting 

 into the blood stream dead or attenuated 

 organisms (vaccines) of the disease against 

 which protection is sought, or in some cases 

 a toxin may be injected. It is also possible 

 to give an injection of an immune blood 

 serum of a person who has had a disease 

 such as measles and has built up immunity 

 through the production of antibodies. If the 

 serum injected is rich enough in the anti- 

 bodies against measles, the disease is usually 

 prevented or at least its severity is much 

 lessened. The substance that is responsible 

 for the development of a certain antibody is 

 called an antigen. The antibodies are pro- 

 tein substances. 



Human blood groups 



In addition to antibodies produced as a 

 result of contact with an antigen there are 



some naturally occurring antibodies, whose 

 production is a normal event in certain in- 

 dividuals, and not the result of exposure to 

 an antigen for which they are antagonistic. 

 These were discovered when it was found 

 in the transfusion of blood from one person 

 (donor) to another (recipient) that the 

 mixing of the bloods could be harmful in- 

 stead of helpful. The harmful effects of 

 some blood transfusions, it was discovered, 

 were due to the clumping (agglutination) 

 of the erythrocytes, which resulted from an 

 antigen-antibody reaction. The first such 

 unfavorable blood reactions investigated 

 were due to what are now known as the 

 A-B-O groups. 



When blood from a donor belonging to 

 the same blood group as the patient (re- 

 cipient) is used, unfavorable reactions do 

 not occur. Incompatibility of bloods results 

 when the serum of one blood contains an 

 antibody which reacts with the antigen in 

 the erythrocytes of another to cause the 

 clumping of the corpuscles. Landsteiner 

 first found that there are two antigens desig- 

 nated A and B in the erythrocytes of man, 

 and that the plasma may contain 2 kinds of 

 antibodies called a and b. This makes pos- 

 sible 4 kinds of blood groups. The antibody 

 a is antagonistic to antigen A erythrocytes, 

 causing them to clump; likewise, antibody b 

 causes clumping of blood corpuscles contain- 

 ing antigen B. Obviously antibody a must 

 be absent from the blood of a person who 

 has the A antigen; and by the same token, 

 antibody b must be absent from the blood 

 of an individual who possesses the B blood 

 antigen. In other words, whatever antigen 

 one has in his erythrocytes, the correspond- 

 ing antibody is absent from his blood serum. 

 The 4 A-B-O blood groups are designated 

 A, B, AB, and O, where O indicates the 

 absence of both antigens. 



In transfusions, because the donor's blood 

 is diluted in the recipient's blood stream, it 

 is possible to ignore the effects of any anti- 

 bodies in the donor's blood. The primary 

 consideration is, what effect will the an- 



