What is an "antigen"?

What is an "antigen"?

An antigen is a protein product that an antibody can be made against. To give you an example of what an antigen-antibody complex is: if you were exposed to the common cold last week then you have the antigen of the common cold circulating in your body. That antigen is a particular protein. The antigen has a three dimensional formation, the antigen looks a certain way. Your body, in an effort to fight that common cold, makes an antibody to block that particular antigen. When you get a vaccine for anything -- yellow fever, flu vaccine, measles, mumps -- you are injected. The vaccine is a harmless antigen of that particular harmful disease and then, in turn, your body makes an antibody, or a blocking protein, against that antigen.