Antigen and Immunizing agent

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

Immunizing agent and antigen are semantically related. In some cases you can replace phrase "Immunizing agent" with "Antigen", this terms are similar.

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

Antigen and immunizing agent are semantically related. You can use "Antigen" instead a noun phrase "Immunizing agent".