The body has immune responses to many things not just diseases. Yes you get a virus or a vaccine you developed antibodies to that disease but the body will also form antibodies to things the immune system sees as foreign. It could be a parasite, it could be a foreign protein that gets transferred by the parasite, or anything else the body sees as foreign.. In my line of work we depend on that immune response to make antibodies which we purify and use to make blood tests for thousands of biomarkers.