An antigen provokes the immune response; an antibody is the response. Think of the antigen as the trespasser and the antibody as the guard your body sends to deal with it, such as a virus carrying antigens that prompt your blood to produce antibodies against it.
The core difference
An antigen triggers the immune system, while an antibody is the protein the immune system makes to attack that antigen.
- antigen — a substance, usually foreign, that triggers an immune response: each virus carries antigens on its surface.
- antibody — a protein made by the immune system to recognise and attach to a specific antigen: vaccines train the body to produce the right antibodies in advance.
How to tell them apart
The easiest anchor is cause and effect: the antigen comes first and causes the antibody to be made. A vaccine works by introducing a harmless antigen so the body learns to produce antibodies before it ever meets the real, dangerous version.
The mix-up tends to show up in health reporting and test results, where headlines about "antigen tests" and "antibody tests" get used loosely. An antigen test checks for the invader itself, currently present in the body; an antibody test checks for the defence your immune system has already built, which can show up even after the infection is gone.
