HORMONES, DRUGS, AND TOXINS 731 



in which the sensitiveness of the tissue has been abolished. It appears that the 

 antigen has fixed itself in such a way as to prevent the action of further doses. 

 Take, next, the above uterus sensitised to three antigens. Treat it first with sheep 

 serum, so as to desensitise it to this antigen. Its sensibility to horse serum 

 is now also very small, but is unaffected towards egg-white. If, however, the 

 uterus be first desensitised to egg-white, it is found to be desensitised to both the 

 others. In another experiment, it was found that, by first treating with sheep 

 serum (p. 189), the sensibility to egg-white was greatly diminished, though not 

 abolished. Further evidence is afforded by the behaviour of the uterus of 

 animals immunised by repeated doses of horse serum. It is, of course, well known 

 that such animals are no longer sensitive to a small dose of the antigen. But the 

 isolated uterus itself still remains capable of anaphylactic reaction. A series of 

 ten injections of horse serum was given, at first at three days' interval, that is, 

 during the "incubation period" of anaphylaxis. On testing the uterus, it was 



FIG. 259. 



IMMUNISED UTERUS SENSITIVE TO SERA OTHER THAN THE ANTIGEN. 

 DESENSITISATION. 



Guinea-pig immunised to horse serum by ten injections. 



Tested for anaphylaxis twenty-four days after the last injection. Perfused. 



A, 0'5 c.c. sheep serum causes contraction and subsequent desensitisation for a further dose at C. 



B, (After change of bath) O'l c.c. horse serum. 



C, (After further change of bath) 0'5 c.c. sheep serum. 

 R, Fresh Ringer solution. 



D, 0'5 c.c. horse serum (the first dose was not large enough to effect complete desensitisation). 



(Dale, 1912, Fig. 12.) 



found to be sensitive to sheep serum, though less so than to horse serum. It was 

 desensitised, however, to sheep serum in the process, so that the phenomenon was 

 a genuine anaphylaxis (see Fig. 259). These facts seem to exclude explanation by 

 specific " chemo-receptors." If there is a particular receptor for egg-white, for 

 example, which is taken possession of in treatment with the antigen, why is the 

 tissue thereby made insensitive to sheep and horse serum also ? 



If three months were allowed to elapse after the last injection of horse serum, 

 although the immunity of the whole animal was still present, the anaphylactic 

 reaction of the excised uterus had nearly disappeared. This latter kind of 

 immunity, by disappearance of sensitiveness, must not, of course, be confused with 

 the immunity conferred by the presence of antibodies in the blood, which are 

 supposed to neutralise the antigen before it has attacked the tissues. 



It has been already mentioned that the blood of an anaphylactic animal can 

 produce the state in a normal one, so that Dale made experiments to see how far a 

 normal uterus could be sensitised by treatment with serum of a sensitised animal. 

 It was found comparatively easy to resensitise an anaphylactic uterus which had 



