Antigen (noun)

1. A substance that is capable of causing the production of antibodies or a T-cell response in the body, especially a protein or polysaccharide that is present on the surface of microorganisms, cancer cells, or other foreign substances, and is recognized as foreign by the immune system.
2. Any substance that can cause an immune response.

Origin:

Early 20th century: from anti- + -gen.

Examples:

1. The flu virus is an antigen that triggers an immune response.
2. The antigen test is used to detect the presence of a specific substance in the blood.
3. The vaccine is designed to train the immune system to recognize and attack a specific antigen.
4. The antigen-presenting cells are responsible for displaying antigens to the immune cells.
5. The antigenicity of a molecule refers to its ability to stimulate an immune response.