The use of tetanus toxoid to elucidate the delayed-type hypersensitivity response in an older, immunized population

Tuberculin reactivity decreases with age despite epidemiologic evidence that the elderly are more likely to have been infected. Whether this phenomenon is due to lack of antigenic stimulus or host inability to mount a delayed type hypersensitivity (DTH) response is unclear. In order to determine if the DTH response to tetanus toxoid in an exposed population is a useful tool to understand the phenomenon of lack of tuberculin reactivity in the remotely exposed elderly, a trial of skin testing was undertaken. Seventy-seven residents of a Veterans Affairs domiciliary were skin-tested using solutions of tetanus toxoid, candida and mumps skin test antigen. The 35 subjects who had negative reaction to the tetanus skin test were randomized into two groups: one which received tetanus vaccination before repeat skin testing and one which did not. Positive skin test reactions occurred in 42 patients to tetanus toxoid, 44 to mumps and 37 to candida. Of the 35 randomized, 27 were available for repeat skin tests. None reacted to the repeat tetanus skin test although 5 reacted to other antigens to which they had previously been nonreactive. Tetanus toxoid was equal to other antigens in its ability to elicit a DTH response originally; however antigenic stimulation with vaccination did not elicit positive skin test in nonreactors. Lack of DTH response to tetanus toxoid in recently vaccinated patients implies that nonresponse was secondary to host factors rather than lack of antigenic stimulation.