antigen

American  

[an-ti-juhn] / ˈæn tɪ dʒən /

noun

1. Immunology. any substance, as a protein, carbohydrate, etc., that, on entering the body, can stimulate an immune response and combine with a specific antibody or T cell receptor having a matching molecular structure.

2. Pharmacology. any commercial substance, usually synthetic, that stimulates the production of antibodies when injected or absorbed into animal tissues.

3. antigens of a particular type collectively.

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antigen British  

/ -ˌdʒɛn, ˈæntɪdʒən /

noun

1. a substance that stimulates the production of antibodies

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antigen Scientific  

/ ăn′tĭ-jən /

1. A substance that stimulates the production of an antibody when introduced into the body. Antigens include toxins, bacteria, viruses, and other foreign substances.

2. Compare antibody See Note at blood type

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Usage

What does antigen mean? An antigen is a substance that causes an immune response in the body—specifically, an antigen causes the body to produce antibodies. Antigens are things like viruses, bacteria, toxins, cancer cells, and other foreign substances, like the cells of a transplanted organ. An antibody is a protein produced by some cells as part of the immune system’s defenses. Antibodies attach to antigens and make them harmless or help the body to destroy them. Antigens are studied in fields like immunology, virology, and pharmacology. Most vaccines contain forms of antigens that trigger the body to produce specific antibodies to fight that antigen.

Other Word Forms

• antigenic adjective
• antigenically adverb
• antigenicity noun

Etymology

Origin of antigen

First recorded in 1905–10; anti(body) + -gen

Compare meaning

How does antigen compare to similar and commonly confused words? Explore the most common comparisons:

• antigen vs. antibody

Explanation

An antigen is a substance that your immune system reacts against. A harmful virus is one kind of antigen. When your immune system detects some toxic foreign body or substance, it sends up defenses against this antigen called antibodies. This ability of your body, to protect you against bacteria and other antigens, helps keep you healthy. Antigen comes from a French word, antigène, from Greek root anti-, "against, and the word-forming suffix -gen, "thing that produces or causes."

Example Sentences

In a Phase 1 trial, 14 of 17 patients showed at least a 50% reduction in prostate-specific antigen, or PSA, a protein that indicates the presence of prostate cancer at elevated levels.

From Barron's • Feb. 24, 2026

It also includes a harmless antigen, an egg protein known as ovalbumin or OVA, which draws T cells into the lungs and helps sustain the boosted innate response for weeks to months.

From Science Daily • Feb. 23, 2026

Eli Lilly said Orna’s lead program, ORN-252, is a clinical trial-ready in-vivo chimeric antigen receptor T-cell, or CAR T, therapy designed to treat B cell-driven autoimmune diseases.

From The Wall Street Journal • Feb. 9, 2026

Using shared features of these viral proteins, they engineered an optimized antigen, the portion of the virus that prompts an immune response.

From Science Daily • Feb. 4, 2026

When the connection is made, and a particular lymphocyte with a particular receptor is brought into the presence of the particular antigen, one of the greatest small spectacles in nature occurs.

From "The Lives of a Cell" by Lewis Thomas

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