p. B. MEDAWAR 



has led to an astonishing prohferation of research, has provided a 

 valuable new analytical tool, and has established important new 

 principles in the field of general immunology, e.g. of the occurrence 

 of immunologically competent cells in peripheral blood. (Here 

 perhaps I should mention a cognate discovery, that lymphocytes, 

 so far from being ephemeral, are long-hved cells that circulate and 

 recirculate through the lymph nodes.) To continue, the past eight 

 years have witnessed the extraction of sensitizing antigens from 

 tissues and the development of a technique of assay, now being 

 enlarged and reinforced by serological methods ; the proof that 

 the allogeneic — "homologous" — bone marrow cells used to re- 

 pair radiation injury behave as grafts, and that a state having much 

 in common with immunological tolerance may arise when irradi- 

 ated mice are so treated; the use of transplantation techniques in 

 the analysis of somatic cellular genetics, particularly of the variants 

 that arise in populations of heterozygous tumour cells; the exten- 

 sion of the principles of transplantation immunity far down the 

 ladder of vertebrate evolution, to amphibia and teleosts; the 

 transfer of sensitivity to homografts in human beings by sub- 

 cellular fractions of blood leucocytes ; the revelation of the ano- 

 malous position of the golden hamster, and the analysis of why it 

 should be so ; the proof that two totally different tissues, skin 

 epithehum and the endocrine component of the ovary, have a 

 quahtatively similar representation of the histocompatibility anti- 

 gens — with the important theoretical consequence that something 

 between ten and fifteen neutral marker genes are present and at 

 work in tissues which, though descended from the same zygote, 

 have followed altogether separate pathways of differentiation; and 

 the demonstration that in guinea pigs a delayed hypersensitivity 

 reaction accompanies the rejection of homografts, a fact which 

 strengthens the analogy between skin homograft reactions and the 

 cell-mediated immunities. There is much else besides— for ex- 

 ample, the slowly growing realization of the great differences 

 between the reactivities associated with "strong" (in mice, with 



