Cross Immunization

CROSS IMMUNIZATION, a method of immunization having the purpose of clarifying the immunobiological relationships between two or more microbes. If cross immunization is performed with microbes A and B, then one group of animals is immunized with a culture of microbe A, and another group with a culture of microbe B. After immunization, the animals immunized with microbe A are tested for the immunity they have acquired by the administration of a lethal dose of microbe B, and conversely, the animals immunized with microbe B are given a culture of microbe A. In addition to testing immunity, cross immunization may also aim to determine the state of antibodies in the immunized animals against the antigens participating in the experiment, i.e., the antigenic properties are studied. Consequently, after immunization, agglutination tests, complement fixation tests, etc., are performed with the sera from group A animals and microbe B, and conversely. With the serum of group A animals, agglutination tests and other immunity reactions are also performed with the specific microbe (A), and the titer is compared with the titer of the reactions with microbe B. With the serum of group B animals, immunity reactions are also performed with microbe B. Conclusions can be drawn from the difference in titers regarding the degree of closeness of the microbes being studied to each other. Example: 10 rabbits are immunized with typhoid bacilli, with weekly intervals. 14-15 days after completion of immunization, they are injected with 1/17 of a lethal dose of a culture of any parashtrain being tested for its relationship to the typhoid bacillus. If the rabbits remain alive, it can be concluded that the typhoid bacillus confers immunity also with respect to the test strain (parashtrain). Simultaneously, agglutination tests and other immunity reactions are performed with the serum of the immunized rabbits and typhoid bacilli on one hand, and the test strain on the other hand. In a parallel series of experiments, 10 rabbits are taken and immunized with the test strain to study its relationship to typhoid fever. After completion of immunization, a comparative study of immunity with respect to the typhoid bacillus and the test strain is performed.