In earlier observations with the picryl system, it was concluded that contact sensitivity was a form of delayed (cellular) hypersensitivity to conjugates of sensitizer with autologous proteins indistinguishable in its immunological mechanism from other classical forms of delayed hypersensitivity to proteins. This conclusion has been confirmed and extended with the picryl and chlorbenzoyl chloride systems.

1. It is shown that to induce a state of contact sensitivity, the minimal necessary amounts of hapten are of the same order of magnitude, whether this hapten is conjugated with protein or the free reactive chemical itself. From this, it is evident that contamination of conjugates with small amounts of unreacted sensitizer plays no part in the induction of contact reactivity by the conjugate. With the dinitrophenyl system, no contact sensitivity could be induced by the conjugates used; possible reasons for this discrepancy are discussed.

2. Animals sensitized to contact by homologous conjugate can be completely desensitized by injections of such a conjugate in large amount; a similar injection schedule has no effect on the contact sensitivity of animals sensitized with the free reactive sensitizer.

3. The capacity of heterologous (ovalbumin) conjugates to evoke anti-hapten antibodies is shown to be greater than that of homologous (guinea pig seralbumin) conjugates: the reverse is true of their capacity to induce delayed reactivity.

4. Evidence is brought forward to suggest that in animals sensitized with homologous albumin conjugates, the specificity of the delayed reaction involves more than the hapten alone, even though the carrier protein is non-antigenic on its own. The contrast with the apparent lesser specificity of the antibodies later produced is discussed.

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