An antigen triggers a response from the immune system and an antibody is produced. All antibodies are immunoglobulins, therefore, the correct answer is immunoglobulin. More specifically, an immunoglobulin is a protein produced by plasma cells to fight and neutralize pathogens. To do so, the immunoglobulin has a specific region which is complementary to a specific region of the antigen. Immunoglobulin's mechanism of action is to bind to the pathogen and either block its action or trigger a response of other cells of the immune system.