While other agents may not cause true immunotolerance, infection by them may not always cause the production of antibodies detectable by the usual tests. Heifers born to cows infected with Brucella abortus may have a latent infection but have no antibodies detectable by the usual tests until near or after parturition. Thus, the absence of antibody is not always assurance that infection does not exist. These tests vary widely in their capabilities, and the titer to an antigen in a serum may be much higher by one test than it is by another. Killed leptospira antigens show much lower titers than do the live cultures used in the microagglutination test.
All serologic tests have deficiencies of one type or another.
It is hardly reasonable to expect practitioners be aware of all these factors, but they should at least know that serologic tests are not infallible, and that the results often depend on the kind of test used. The antibody titer that results from vaccination varies with the antigen, the adjuvant, the recipient species, previous and subsequent exposure to the antigen, and the individual's immunologic reaction to the antigen. Because these and other factors affect the titer and its persistence, it is extremely difficult to interpret the results of serologic tests on vaccinated animals. In addition to the difficulties this presents in using antibody titers in clinical diagnosis, antibody titers are not always directly related to resistance to infection. Some animals that have no detectable antibody to an infectious agent may still be resistant to infection by that agent.