When a foreing or toxic substance enters the body, it induces an immune response, especially the production of antibodies. So antibodies are a consequence of what is causing the specific immune response, therefore A is incorrect.
Prostaglandins are known for its hormone-like effects, like its action in uterin contractions. Therefore, B is also incorrect.
Plasma proteins are all of the proteins that are located in the plasmic membrane, therefore C is incorrect as well.
Antigen is the name given for a substance that enters the body and causes a specific immune response. Therefore, the correct answer is D. antigens.