Antigen

If an antibodyAntibodyAn antibody is a protein made by the immune system that locks onto one specific target on a germ and marks it for destruction. Antibodies are what give a person lasting protection after an infection or a vaccination.Read the full definition is a key, the antigen is the lock. It is typically a protein or a sugar sitting on the outside of a bacteriumBacteriaBacteria are single-celled living organisms, most of them harmless or useful, a minority of which cause infection. Unlike viruses they can be killed by antibiotics, which is the whole reason the distinction matters at the point of treatment.Read the full definition, a virusVirusA virus is genetic material in a protein coat that can only reproduce inside a living cell. Antibiotics have no effect on viruses, which is why they are not prescribed for a common cold.Read the full definition or a parasite — distinctive enough that the immune system can tell it apart from the body's own material. The immune response is built around this specificity: the body does not learn germs, it learns shapes.

Where the word turns up on a report

• Rapid antigen tests look for the germ's own material in a swab or a drop of blood — the dengue NS1 test and rapid malaria tests work this way. A positive result means the germ is present now, which is why these are used early in an illness while antibody tests are still negative.
• HBsAg, hepatitis B surface antigen, means active hepatitis B infection. The antibody to the same antigen, anti-HBs, means the opposite: immunity, from recovery or from the vaccineVaccineA vaccine teaches the immune system to recognise a specific germ before it is ever encountered, so that a real infection is met by defences that are already prepared. It is prevention, not treatment.Read the full definition.
• PSA, prostate-specific antigen, is a protein the prostate makes normally. Here the word simply means a recognisable protein; it has nothing to do with infection.
• Blood group antigens are the A and B markers on red cells. They are why a transfusion has to be matched.

Vaccines exploit antigens directly. A vaccineVaccineA vaccine teaches the immune system to recognise a specific germ before it is ever encountered, so that a real infection is met by defences that are already prepared. It is prevention, not treatment.Read the full definition presents the antigen alone — killed, weakened, purified or made to order from a genetic instruction — so the body builds antibodies and memory cells without ever meeting the disease. When the real germ arrives, the shape is already known and the response is fast enough to prevent illness rather than merely survive it.