SESSILE AND HUMORAL ANTIBODIES 227 



proposition that selection of antibody-resistant cells was the basis 

 of enhancement. He found that an inoculum of "enhanced" 

 Sa I injected into C57BL/K hosts was destroyed if such hosts had 

 been preimmunized 7 days previously, but identical preimmuni- 

 zation 14-28 days previously led to yet greater enhancement. 

 Boyse in unpubhshed work at this laboratory has subjected 

 enhanced mammary tumours, dissociated by an enzymic method, 

 to cytotoxic tests and found that no change in population 

 susceptibility to isoantibody could be associated with enhanced 

 growth. 



There are rather more data on modification of the host response. 

 The afferent blocking theory (Snell, 1956; Billingham, Brent 

 and Medawar, 1956) originally proposed that appropriate antigens 

 do not reach the host lymphatic system in sufficient quantity to 

 immunize. In a recent paper (Snell et al, i960) Snell and co- 

 workers described how passive immunization of B10.D2 hosts 

 grafted with Sa i inhibited the rate of development and the 

 level of immune activity attained by the draining lymph nodes. 

 To explain Kaliss' demonstration that enhancement may result 

 from serum injected up to 10 days after graft implantation 

 (KaHss, 1958), Snell and colleagues suggest that the maintenance 

 of a high level of cellular hypersensitivity requires a constant 

 supply of antigen. Inhibition of immunization, particularly when 

 following the administration of large doses of antibody as was 

 the case in Snell's experiments, may well be one factor; but it 

 cannot provide a full explanation. Enhancement is one of the 

 most sensitive methods available for the detection of isoantibody, 

 and the small amounts of serum required for its induction do not 

 prevent the initial stages of the host response from occurring. 

 Both Gorer's histological evidence (Gorer, 1958) and our own 

 data show that host reactive cells invade the graft site in normal 

 quantities but their destructive function is incompletely fulfilled. 

 Later they may disappear from the graft site. The results we 

 obtained suggest that sensitized cells are unable to function with 



