Antigen and Immunogen

Antigen noun - Any substance (as a toxin or enzyme) that stimulates an immune response in the body (especially the production of antibodies).

Immunogen and antigen are semantically related. In some cases you can replace term "Immunogen" with "Antigen", this nouns are similar.

Immunogen noun - Any substance or organism that provokes an immune response (produces immunity) when introduced into the body.

Antigen and immunogen are semantically related. You can use "Antigen" instead a noun "Immunogen".