Immunological Studies of Anthrax. III. Comparison of Antibody Titer and Immunity Index Following Anthrax Immunization

The effects of anthrax immunization on the serum antibod titer and the immunity index were compared for guinea pigs and rats immunized by two protocols. An association between the two measures could not be detected in either species of animals immunized at any one level; however, when data on animals immunized at several levels were combined a weak but significant association became apparent. The results of the agar diffusion test were more variable than those of the immunity index, and at relatively low or intermediate levels of protection the agar diffusion test produced a much larger portion of false negatives. This weakness did not carry through to animals protected to a high level of resistance, although at nearly all levels of protection the immunity index was more precise than the serological test. A comparison of three antigens administered by two protocols showed differences in both titer and index attributable to antigen. In this study only the Belton and Strange antigen significantly increased both the immunity index and the serum antibody titer.