IMMUNOLOGICAL COMPETENCE OF SMALL LYMPHOCYTES 255 



ill homologous newborns, it is apparent that 800,000 or more 

 small lymphocytes are required to produce the same reactions. 

 The most likely explanation for this difference is that spleen and 

 lymph nodes contain many cells that multiply in the host, whereas 

 the small lymphocytes derived from adult blood fail to do so. 

 On the basis that each immunologically competent cell injected 

 will cause a certain quantum of damage to the neonatal host, one 

 might suppose that death would be hastened at progressively 

 higher cell doses. This was not found over the dosage range 

 tested with lymphocytes from unimmunized donors. However, 

 a moderate dose of 900,000 small lymphocytes from animals 

 specifically presensitized to host antigens did result in a significant 

 acceleration of the runting syndrome with early death. This 

 fmding strongly supports the contention that the syndrome 

 mediated by small lymphocytes has a primary immunological 

 basis. 



The capacity of isologous, adult A/Jax lymphocytes to protect 

 newborns injected with C57BL/6 lymphocytes has not yet been 

 tested. Such protection with respect to spleen cells in this strain 

 combination has been demonstrated by Siskind, Leonard and 

 Thomas (i960). 



A median survival time of 14' 9 (13 • 7-16-4) days was calculated 

 from Russell's (i960) data on the time distribution of deaths of 

 39 C57BL/6 mice that received one to two milhon DBA/i 

 spleen cells intravenously during the day of birth. Although the 

 strain combination was different from ours and the ultimate dose 

 of cells probably very high, the MST determined is essentially the 

 same as we found (c£ Table I). Thus it may be inferred that the 

 time course of fatal transplantation disease is much the same 

 whenever (i) a threshold dose of immunologically competent 

 cells is exceeded in newborn, homologous hosts and (2) strong 

 antigens exist in host cells that are lacking in the donor. Even in 

 Fi (C57 X A) hybrids inoculated at three weeks of age with very 

 high doses of C57 or A spleen and lymph node cells, Trentin 



