Clinical reactions following administration of penicillin have been noted since this antibiotic was first used (1, 2). These side effects are considered to be allergic, and both serological and cutaneous tests (3, 4) confirm the immunological activity of the drug. However, the characteristics of this compound as an antigen have not yet been defined.Because of its low molecular weight, the binding properties of penicillin are important in its allergenic potential, since other compounds of similar size which possess antigenic activity have been shown capable of firmly coupling with large molecules in vitro and in vivo (5). Penicillin and serum albumin combine in a loose association (6), but this type of binding has been demonstrated to be ineffective in making other low molecular weight substances antigenic (7). The important combination has not been determined. Another unresolved question is that of the specificity of the immunological response, including the cross-reactivifies of the various types of penicillin. These two problems, it was felt, could best be investigated in the experimental animal.In this report the production of antibodies to penicillin in the rabbit is described, and data bearing on the binding capacity of penicillin and the specificity of the immunological response are presented.
Materials and MethodsImmunization Te~hnifue.--White rabbits of either sex and 2500 gm. at the start of the experiment were fed Purina chow and water ad libltum. Commercial aqueous penicillin G potassium (Lilly and Pfizer) was used for inoculation with complete Fretmd's adjuvant (Difco). Basic immunization procedure extended over a 4 week period. At the start of the 1st week each animal received 100,000 units of penicillin dissolved in 1 ml. of saline and emulsified with an equal volume of adjuvant. One-tenth ml. of the emulsion was injected into each foot-pad, and the balance injected intradermally in sixteen areas over the back. The second and third injections were performed at weekly intervals and consisted of the deposition of a similar quantity of emulsion, subcutaneously, usually in two separated areas. At the start of the 4th week each animal received 0.1 ml. of the emulsion in each foot-pad and 1.6 ml. subcutaneously. All animals were bled prior to the start of the procedure, 7 days after the last injection and at