Confused About Antigen Testing For COVID-19? Here's How It Differs From Antibody Testing

The novel coronavirus has added a lot of terms to our collective vocabulary. The latest is antigens, and though these proteins may sound similar to antibodies – and both will be key to test as doctors learn more about COVID-19 – they actually play very different roles in the body and the course of the disease. POPSUGAR asked experts for a quick breakdown of these two terms and what they mean for the strategy moving forward.

What Is an Antibody?

Contrary to what it might sound like, antibodies actually protect your body! Your immune system produces these little Y-shaped proteins to defend you against invader cells (pathogens). “An antibody is the immunoglobulin (protein) produced by the immune system,” Habib Sadeghi, DO, a physician and integrative medicine specialist in Los Angeles, told POPSUGAR.

Fun fact: antibodies are produced by certain lymphocytes. A doctor can check your antibody levels in your blood. For COVID-19, “doctors look for two kinds of antibodies to SARS CoV-2 to determine exposure: IgM antibodies that develop early in an infection and IgG antibodies that appear after levels start to drop about four weeks in,” said Dr. Sadeghi. The latter would in theory allow you to develop some level of long-term immunity, but experts still aren’t sure if that’s the case.

Related: Study Says Shutdowns Put in Place Avoided 60 Million More Coronavirus Infections in the US

What Is an Antigen?

Antigens are not produced by the body. “An antigen is any substance that induces a response from the immune system,” Dr. Sadeghi explained. This can range from a toxin to simply a foreign substance. Remember the pathogens we just mentioned? Those invader cells? Antigens are part of pathogens – the part that triggers the antibodies to spring into action.

From there, “an antibody is capable of binding with the antigen and neutralizing it,” Dr. Sadeghi said. This is typically how your body fights off an infection.

Related: Why You Should Take Symptoms of COVID-19 Seriously, Even If You’re Not Running a Fever

What Does This Mean For COVID-19?

Antigen tests are the latest offering in the fight against COVID-19, designed to help manage the large number of tests needed to detect active cases. Antigen testing is cheaper than nasal swab testing, and blood results come back faster.

“The antigen test looks for actual pieces of protein of the virus itself, where the antibody test looks for evidence of a person’s immune system response to being infected,” William Kimbrough, MD, of One Medical, told POPSUGAR. “This means that the antigen test is identifying people with active infections (similar to what the more broadly available PCR swabs do), where the antibody tests look for people who have been previously infected.”

Your doctor can help you determine which test is right for you. Just remember: a positive result on the antibody test does not necessarily mean you have immunity, so please, keep social distancing and following CDC guidelines until there’s a safe and effective vaccine.

POPSUGAR aims to give you the most accurate and up-to-date information about the coronavirus, but details and recommendations about this pandemic may have changed since publication. For the latest information on COVID-19, please check out resources from the WHO, CDC, and local public health departments.