ROLE OF THE THYMUS 393 



whether classical immunological tolerance, immunological 

 unresponsiveness, or even Felton's immunological paralysis, 

 appears to be the Junctional absence of antibody-forming cells 

 necessary for reacting with the particular antigens concerned. The 

 physical absence of antibody-forming cells seems to characterize 

 the neonatally thymectomized mouse. It would be satisfying to 

 think that when one is inducing a state of immunological tolerance 

 one is in effect performing a thymectomy, not a total thymectomy, 

 nor a thymectomy in the surgical sense, but a partial, selective, 

 functional thymectomy. In other words, injected cells or antigens 

 might make contact with certain cell types differentiating in the 

 thymus and in some way prevent these cells from maturing to a 

 stage when they would be capable of reacting immunologically. 



To gain support for such a theory, it would be necessary to 

 show that injected cells or antigen can indeed fmd their way to 

 the thymus, that different immunological faculties mature at 

 different times, that immature cells are much more susceptible to 

 functional elimination than mature cells, and that cells actually do 

 migrate out of the thymus to other tissues. 



There are some well documented data which suggest that 

 nothing that is injected into a rodent ever fmds its way into the 

 thymus. For instance, Gowans, Gesner and McGregor (1961) 

 showed that tritium-labelled small lymphocytes injected into a 

 rat exhibit what is generally known as a homing instinct. That is, 

 they settle in the lymph nodes, in the white pulp of the spleen, in 

 the Peyer's patches and in the bone marrow. They cannot, how- 

 ever, be found in the thymus. Marshall and White (1961) showed 

 that trypan blue or pneumococcal polysaccharide or other antigen 

 when injected parenterally into guinea pigs was not taken up by 

 the normal thymus. These studies suggest the existence of a 

 barrier against the entry of cells or antigenic material into the 

 thymus. Reasoning teleologically, one can appreciate the signifi- 

 cance of such a barrier, if it is conceded that immunologically 

 competent cells are differentiating in the thymus even in postnatal 



