Antibody Anti-JKa - What is it and how does one get it?

I gave blood a few weeks ago and today I received a letter from the blood center telling me I tested positive for an alloantibody called anti-Jka. It says "These antibodies may be present in the body for several reasons, such as previous pregnancies, transfusions, or other types of immunization." I've never been pregnant and as far as I know i'm currently not pregnant. I've never had a transfusion, and I haven't had an immunization for a year or so. Can anyone tell me what it is and what it means that I have it?