THE FACTS OF IMMUNITY 



serum from the reactive animal fails, but by the use of cells, 

 apparently lymphocytes, passive transfer of skin reactivity 

 usually succeeds. 



The three types of immune response can be conveniently 

 called : 



(i) Classical antibody production, 



(2) Hay-fever type response, 



(3) Tuberculin type response. 



It might be equally legitimate to add as a fourth type 

 of immune response the development of specific immune 

 tolerance when by Medawar type manipulations a specifically 

 limited failure to give the expected normal immune response 

 of types (i), (2) or (3) is induced. This is essentially a negative 

 type of finding but it does demand, on some theories at least, 

 a positive process by which non-reactivity is induced. 



(i) Under classical antibody we include all serum com- 

 ponents which react specifically with homologous (and in 

 some cases related) antigens by the production of visible 

 aggregation on mixing solutions under appropriate conditions. 

 Within this definition there is still scope for inclusion of quite 

 different types of antibody; for example, it includes both 

 low-molecular-weight (±160,000) and high-molecular- 

 weight globulins ( + 900,000) . In addition, there is much to 

 suggest that under certain circumstances antibody molecules 

 may be produced which show a wide range in avidity or 

 demonstrability but which do not appear to be qualitatively 

 different. It is common, for example, to find in pathological 

 conditions cold agglutinins for certain types of erythrocyte 

 in which agglutination disappears on warming but with 

 intensification of the disease process warm agglutinins, pre- 

 sumably of higher avidity, appear. At a still lower level are 

 the incomplete antibodies which do not cause aggregation 

 but can be recognized by a positive Coombs test. Here the 

 situation is coming close to that of hay-fever type antibody. 



It is a moot point whether or not 'natural antibodies' 

 should be included in the class of classical circulating anti- 



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