Antigens

An antigen is a substance that can stimulate an immune response. It’s usually a protein found on the surface of cells, bacteria, viruses, and fungi.
Antigens are recognised as foreign by the immune system, triggering the production of specific antibodies to fight against it.
Different pathogens have different antigens, therefore each triggers a unique immune response.
Some antigens can mutate or change over time, making it harder for the immune system to recognise and attack them - an example of this is the flu virus.
Vaccinations work by introducing small amounts of weakened or dead pathogens into the body. These contain antigens that stimulate the immune system to produce antibodies, preparing it to fight the actual pathogen if it ever enters the body.
The ability of our immune system to remember the antigens it has encountered before is what gives us immunity. The next time the same antigen enters the body, the immune system is able to respond more quickly and effectively.
Antigens not only elicit an immune response but are also targets of the response. The produced antibodies can either neutralise the antigen directly or mark them for destruction by other cells of the immune system.
The production of antibodies following the recognition of an antigen is a part of the body’s adaptive immune response.
Antigens can also be used in medical testing. For instance, a pregnancy test works by detecting the presence of the HCG antigen in urine.