Concluding remarks

In many ways immunological tolerance is an ideal subject for discussion at the present time. Experimental work has gone far enough to allow us to claim that the principle of immunological tolerance is soundly established and that we can see more or less clearly some of its implications. But obviously very much remains to be learnt of the part played by tolerance in the various fields that have been discussed. It is by no means certain that we are dealing with a single topic when we compare tolerance to homografts with inhibition of antibody production against soluble protein in a rabbit. Such a situation provides much for discussion but does not make it easy to condense or interpret that discussion. One might begin by reiterating that immunology is concerned with much more than the production and properties of typical circulating antibody. There are at least four different types of immunological reaction and there are hints of many minor differences within the main types. Pappenheimer’s recent work on the variety of responses given by a single species, man, to a single purified antigen, diphtheria toxoid, offers a characteristic example of the current trend. Chase’s experiments on the response of guinea pigs to simple allergens like picryl chloride, have been only incidentally mentioned in today’s discussion, but their importance is obvious. A form of tolerance very similar to that produced by prenatal treatment of mice can be produced by administering the allergen to adult guinea-pigs by mouth. The animals are resistant to sensitization by skin treatment and the inhibition is general and unrelated to any persistence of allergen in the body. The question immediately arises whether all forms of tolerance are basically similar or whether for each of the qualitatively distinct types of positive immunological reaction, a correspondingly distinct type of inhibition or tolerance must be sought.