Final answer:
Antibodies produced by a vaccine are specific to the antigen of the targeted pathogen. A vaccine for one disease, such as hepatitis A, is ineffective against a different virus like measles because the antigens are different, and the antibodies can't recognize and bind to them.
Step-by-step explanation:
The reason why antibodies produced by a vaccine against one infectious agent, such as hepatitis A virus, do not protect against a different infectious agent, like the measles virus, is because antibodies are highly specific in their action. Each antibody is designed to recognize and bind to a unique antigen, which is a molecule or part of a molecule on the surface of the infectious agent. This specificity stems from the unique three-dimensional structure of the antigen-binding site of the antibody. The immune system's precision ensures that each vaccine generates antibodies that are tailored to the specific pathogen.
However, this specificity also means that antibodies against one disease are ineffective against other diseases with different antigens. For example, the influenza virus has a high mutation rate, causing it to change its surface molecules frequently. This makes an influenza vaccine from one season ineffective in subsequent seasons, as new vaccines need to be developed to match the mutated strains. In contrast, the viruses causing measles, mumps, and rubella mutate infrequently, making vaccines against them effective for longer periods.