An antigen is a foreign protein (such as bacteria or virus). I called it foreign because it is not a normal part of your body. Your cell membranes are designed to recognize that those substances are foreign.
When such a recognition occurs, a series of chemical reactions occur that result in your body GENerating ANTIbodies. Thus, foreign proteins are called ANTIGENS because they stimulate your body to generate antibodies.
In many situations, once you are exposed to an antigen, your body will continue to make antibodies for, up to, your life. Thus, you become immune to the antigen.
So, antibodies fight off antigens.