Doctor Decoded: What Are Antigens?

You’ve probably heard of an COVID-19 antigen test. What is that?

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COVID-19 has introduced a number of medical terms and new phrases to casual conversations. Since early 2020, you've learned words like social distancing, cloth face coverings, respiratory droplets, and cytokine storm. Another new phrase you may have heard is COVID-19 antigen tests. This may lead you to wonder: What are antigens?

Your Immune Response

Antigens are substances on the surface of cells, viruses, bacteria, or fungi. They’re often proteins—but not always. When your immune system detects an antigen, it knows there’s a threat to the body. It tells your immune system to launch an attack, which helps protect your body from harm.

(FYI, when your body fights antigens, it produces antibodies to fight off the threat. Learn more about what antibodies are here.)

Not all antigens are dangerous. You have millions of them on your own red blood cells. Your immune system learns to recognize these as safe, so it ignores them. That’s why it’s important for blood donors to “match” antigens with the recipient. If they don’t match, the recipient's immune system will attack, and their body will “reject” the blood donation. (Learn more about blood types here.)

What Are COVID-19 Antigens?

COVID-19 antigens are proteins of the SARS-CoV-2 virus. (This is the official name for the virus that causes COVID-19.) Your immune system accurately recognizes them as harmful, so it launches an attack on the virus.

Thus, COVID-19 antigen tests work by looking for fragments of these proteins. If the test detects any, the test results will likely be positive. This means you probably have the infection. (Learn more about what a positive COVID-19 test means here.)

Antigens get a bad reputation, but they play an important role. They help your immune system detect threats, which helps keep you safe.