510 



ORGAN SYSTEMS OF MAN 



fact, this is the only way we have of sepa- 

 rating and classifying specific proteins, for 

 it is more delicate than any chemical tests 

 known. Proteins are so complex that the 

 structural formula has never been written 

 for one of them. There are probably mil- 

 lions of them, and each animal possesses 

 its own specific proteins for which there is 

 no chemical means of separation. Using an 

 animal for a test tube, it is possible to de- 

 termine by experimentation the many dif- 

 ferent kinds in a very precise manner, even 

 to determining from what animal a bit of 

 blood or tissue originated. This has many 

 practical applications, such as identifying 

 whether or not a specimen of blood is from 

 man or a lower animal. This is done by 

 withdrawing the blood from an animal that 

 has previously been injected with a foreign 

 protein, for example, egg albumin, and then 

 separating out the cells by whirling the 

 blood in a centrifuge (an instrument for 

 increasing gravity). If some of the original 

 albumin is then added to this clear serum, 

 a white precipitate will form. This reaction 

 will occur for only one protein and no oth- 

 ers; it is, therefore, highly specific. 



One interesting use of this reaction was 

 employed by Nuthall of England years ago 

 in the classification of animals. He built up 

 antibodies in experimental animals against 

 the blood of other animals. For example, he 

 might inject the blood of the ape into a rab- 

 bit and after a time draw off a sample of the 

 blood from the experimental animal, cen- 

 trifuge out the cells, and to the clear serum 

 add some of the serum (antigen) of a 

 closely related animal, say a gibbon. He dis- 

 covered that the closer tlie animals were 

 related, according to the usual system of 

 classification, the heavier was the precipi- 

 tate that formed. In this case, the serum of 

 the gibbon would give a rather heavy pre- 

 cipitate, whereas the serum from a pig 

 would give much less, and that from a 

 snake none at all. Interestingly enough, he 

 was able to confirm the classification based 

 on morphological structures. 



The graded precipitate of the precipitin 

 reaction, as this is called, is a distinct value 

 not only in the relationships pointed out 

 in tlie preceding paragraph but in deter- 

 mining the closeness of the chemical rela- 

 tionship of different proteins. 



Undoubtedly antibodies are formed in 

 tissues other tlian the blood, but just where 

 is not known. They do appear, however, in 

 the plasma, and since this is a readily avail- 

 able tissue, most of our knowledge of anti- 

 body formation has come from studies of 

 the blood. It might be thought that anti- 

 body formation could occur in a test tube. 

 What an excellent way to prepare it, if this 

 were true! Unfortunately, it forms only in 

 the living organism. Where it has practical 

 significance, as in preventing certain dis- 

 eases, it is produced in large animals such 

 as the horse. 



Blood types 



From the foregoing discussion on the re- 

 latedness of animals, it might be expected 

 that all animals of the same species have the 

 same specific proteins. In general this is 

 probably true. However, there are minute 

 variations of which we have become aware 

 primarily through our efforts to transfuse 

 blood from one person to another. They 

 have also been called to our attention in 

 plastic surgery, where attempts are made 

 to graft tissues from one person to replace 

 the destroyed tissues of another, as in case 

 of severe burns, for example. One wonders 

 why the healthy, intact organs of people 

 dying in accidents could not be saved and 

 transplanted in the bodies of those who are 

 dying because the same organs are no longer 

 functioning properly as a result of some 

 organic or infectious disease. So far we have 

 been limited to blood and corneas of the 

 eyes in this regard, primarily because of 

 the specificity of proteins of individuals. 

 In other words, the proteins of one person 

 are slightly different from those of another, 

 so that such transplants are incompatible 

 and will not "take." However, even with 



