14 BRENT, MEDAWAR AND RUSZKIEWICZ 



prolong the life of homografts, but no firm reliance can be placed 

 on this finding at present. Table III makes it clear that the insoluble 

 antigenic sediment does sensitize when injected intravenously, 

 though less effectively than via the intraperitoneal route; it also 

 shows that the heparin (lOO anticoagulating units per mouse) 

 added as a precautionary measure to the material injected intra- 

 venously may itself weaken the sensitizing action of the antigen 

 it accompanies. 



Clearly an antigen which can be administered in a form and by 

 a route which does not sensitize opens up new possibilities of 

 subverting the homograft reaction, and for this reason we urgently 

 await the preparation of a truly soluble antigen of high potency. 

 It is tempting to think that our results may have some bearing on 

 a phenomenon described by Billingham and Sparrow (1955) — the 

 remarkable prolongation of the life of skin homografts in rabbits 

 secured by the intravenous injection of living dissociated epidermal 

 cells (see also Albert and Lejeune, 1959). Epidermal cells injected 

 into the blood stream presumably remain within the vessel walls, 

 and might there liberate, in a soluble form, antigens which are 

 known to be highly potent (Lejeune and Albert, i960; Berrian 

 and McKhann, i960). However, two reservations should be 

 borne in mind: (i) the absence of response to soluble "transplan- 

 tation antigens" injected by the intravenous route might be a 

 peculiarity of the mouse (cf. the work of Dresser, 1961), though 

 there are certain reasons for thinking it may not be; and (2) the 

 phenomenon may in any event be a trivial one, and depend 

 merely upon the specially rapid degradation of antigen entering 

 through the intravenous route, instead of along a pathway that 

 gives direct access to lymph nodes. 



Extraction with lipid solvents 



The solubility and physical behaviour of sensitizing antigens are 

 obviously affected by the high lipid content of existing prepara- 



