antigen

noun

an·​ti·​gen ˈan-ti-jən 

-ˌjen

: any substance (such as an immunogen or a hapten) foreign to the body that evokes an immune response either alone or after forming a complex with a larger molecule (such as a protein) and that is capable of binding with a product (such as an antibody or T cell) of the immune response

antigenic

ˌan-ti-ˈje-nik 

adjective

antigenically

ˌan-ti-ˈje-ni-k(ə-)lē 

adverb

antigenicity

ˌan-ti-jə-ˈni-sə-tē 

noun

Did you know?

An antibody is a protein produced by your immune system to fight outside invaders. Since the enemy substance actually triggers the production of antibodies, such substances are called antigens—anti- being short for antibody, and -gen meaning "producer". (In a similar way, an allergen produces an allergy, and a pathogen produces a pathology or disease.) Antigens are often rodlike structures that stick out from the surface of an invading organism—usually a bacterium or a virus—and allow it to attach itself to cells in the invaded body. But unfortunately for them, in doing so they let the immune system know they're present, and the body is flooded with an army of Pac-Man-like antibodies.

Examples of antigen in a Sentence

Recent Examples on the Web

During an infection, foreign particles, known as antigens, circulate in our bodies. —Jerome Groopman, New Yorker, 6 Apr. 2026

While some cancers have common antigens, mRNA technology could make personalized vaccines a reality. —Alix Martichoux, The Hill, 5 Apr. 2026

Scientists at the University of Chicago Comprehensive Cancer Center developed a CAR-T platform that can remove and replace the antigen-recognizing portion of CAR-T in vivo, which could better protect a patient’s immune system. —Amanda Erickson, STAT, 27 Feb. 2026

My nurse practitioner ordered a PSA test, or protein specific antigen, which measures a chemical produced by the prostate in your bloodstream. —Jerry McCormick, USA Today, 8 Feb. 2026

Word History

Etymology

German, from French antigène, from anticorps antibody + -gène -gen

First Known Use

1908, in the meaning defined above

Phrases Containing antigen

• antigen-presenting cell
• carcinoembryonic antigen
• human leukocyte antigen
• prostate-specific antigen

Kids Definition

antigen

noun

an·​ti·​gen ˈant-i-jən 

-ˌjen

: a substance (as a protein) that causes the body to form antibodies against it when it is introduced into the body either alone or as part of a microscopic plant or animal

antigenic

ˌant-i-ˈjen-ik 

adjective

Medical Definition

antigen

noun

an·​ti·​gen ˈant-i-jən 

: any substance (as an immunogen or a hapten) foreign to the body that evokes an immune response either alone or after forming a complex with a larger molecule (as a protein) and that is capable of binding with a product (as an antibody or T cell) of the immune response

antigenic

ˌant-i-ˈjen-ik 

adjective

antigenically

-i-k(ə-)lē 

adverb