336 - Multicellular Animals, Especially Man 



tralian worker, has reported analogous evi- 

 dence in birds. In the bird, however, another 

 enigmatic lymphoid organ, the bursa of 

 Fabricius, collaborates with the thymus in 

 influencing antibody production. 



Thymectomy in an adolescent or an adult 

 mouse has very little, if any, physiological 

 effect. But if the thymus is removed at birth 

 — and it requires great skill to excise this 

 pinhead-sized body from a newly delivered 

 mouseling — the consequences are remark- 

 able. The mouselings develop quite normally 

 lor some 3 weeks, but soon thereafter they 

 are prone to sicken and die; and the cause 

 of death is usually some common infection. 

 Moreover, further tests reveal that the anti- 



body, or gamma globulin, fraction (p. 505) 

 is virtually absent from the plasma of such 

 thymectomized individuals. 



Another important observation is that such 

 thymectomized individuals are able to accept 

 skin grafts, not only from themselves, but 

 also from unrelated strains of mice and even 

 from a rat (Fig. 17-18). Such successful hetero- 

 transplants are most unusual. Ordinarily the 

 antibody mechanisms of the animal lead to a 

 rejection of the foreign tissue shortly after it 

 has started to grow, although homotrans- 

 plants from one site to another in the same 

 individual (or transplants from an identical 

 twin) are generally successful. Thus again the 

 conclusion seems to be that the immuno- 



Fig. 17-18. Removal of the thymus gland from a mouse 1 to 3 days after birth leads to a severe impair- 

 ment of the immunity responses. This postnatally thymectomized mouse, tested at the age of 5 weeks, 

 accepted and maintained two foreign skin grafts: (1) from an unrelated mouse (strain C3H), as is shown 

 by the patch of black hair, which grew in quickly; and (2) from a white rat (Wistar strain), as is shown by 

 the white patch. Normally, of course, in a nonoperated animal, vigorous immune reactions would have led 

 to a rejection of these heterografts. (Courtesy of Jacques F. A. P. Miller, The Chester Beatty Research In- 

 stitute and the New York Academy of Sciences.) 



