THE FACTS OF IMMUNITY 



spicuously can often be studied most effectively by analysing 

 what happens when things go wrong. In fact most of the 

 important leads in this field have come from clinical medicine 

 and haematology. It was the complications of blood trans- 

 fusion and later the unravelling of the results of interchange 

 between maternal and foetal bloods that provided a clear 

 understanding of the importance of 'self substance. Plastic 

 surgery has shown that if physiological requirements are 

 fulfilled, parts of the body can be transplanted to other parts 

 without disturbance but that most attempts to transplant 

 tissues from any other individual (except an identical twin) 

 are completely unsuccessful. Combining chnical experience 

 with experimental studies in animals, the position can be 

 summarized by saying: 



(i) All the body components which either directly or as 

 a result of the break-up of expendable cells can be present 

 in blood or lymph are non-antigenic to the individual 

 himself 

 (ii) Cells of different genetic type which have the capacity 

 to implant during embryonic life may also become 

 non-antigenic and survive indefinitely. There are well- 

 known examples from twin studies in cattle and humans, 

 and in recent years Medawar's work on immunological 

 tolerance has drawn wide general attention to this 

 potentiality (Billingham, Brent and Medawar, 1956). 

 (iii) There are body components which can be shown to 

 be antigenic for the individual providing them. In 

 mammals there are thyroglobulin (Roitt et al. 1956) 

 and components of the lens and of spermatozoa. This 

 antigenicity is usually related to the two factors {a) that 

 the components in question are normally inaccessible 

 to blood and lymph and/or (b) they were not present 

 as such in the body when tolerance to self components 

 was established during embryonic life. 

 (iv) Blood, tissue cells or tissue extracts from animals not 

 genetically identical with the host will provoke some 



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