272 H. S. LAWRENCE ET AL. 



antibody-like reagent currently available for analysis of the delayed 

 tuberculin-type of hypersensitivity. 



Discussion of mechanisms of homograft rejection are helped by 

 a definition of the exact type of homograft under consideration. 

 Gorer's (1956) summary of available evidence makes it unwise, if 

 not impossible, to expect the same mechanism to be operative in 

 the destruction of all type of homografts. We will Hmit ourselves 

 to the response evoked by the orthotopic skin homograft — an 

 organized tissue characterized by the estabhshment of vascular and 

 lymphatic connexions with the host. This is in contrast to res- 

 ponses evoked by leukotic cells or by ascites tumour cells. 



Transfer factor and delayed bacterial sensitivity 



In order to place the transfer of homograft sensitivity in human 

 beings in perspective, it is appropriate to consider briefly the 

 events leading up to this extension of the biological properties of 

 transfer factor. As an outgrowth of Chase's (1945) observation 

 using viable cells in animals, there has been found in extracts of 

 human leucocytes obtained from donors with delayed tuberculin- 

 type hypersensitivity a factor (or factors) which when injected 

 into a non-sensitive recipient causes him to respond as the donor 

 to the specific delayed hypersensitivity transferred. The altered 

 reactivity produced in the recipient by this means is prompt, 

 widespread and enduring. The agent involved in the transfer 

 of immunological information has been termed "transfer 

 factor", with the reaHzation that one or more factors may be 

 involved in the transaction (Lawrence, 1960^). Extracts of human 

 peripheral blood leucocytes containing transfer factor have served 

 to transfer delayed-type hypersensitivity to a variety of bacterial 

 (tubercuUn, streptococcal (Lawrence, 1955), diphtheria toxoid 

 (Lawrence and Pappenheimer, 1956)) and fungal (coccidioidin 

 (Rapaport et ah, 1960a)) antigens. Confirmation and extension of 

 the fmdings on the efficacy of non-living extracts of leucocytes 



