Merriam-Webster Collegiate Dictionary - antigen

 noun  Etymology: German, from French antigene, from anticorps antibody + -gene -gen  Date: 1908 any substance (as an immunogen or a hapten) foreign to the body that evokes an immune response either alone or after forming a complex with a larger molecule (as a protein) and that is capable of binding with a product (as an antibody or T cell) of the immune response; specifically immunogen  • ~ic adjective  • ~ically adverb  • ~icity noun