What is the p24 antigen?

In the world of HIV, the discovery of the p24 antigen has been a game-changer for effective HIV infection detection.

This one antigen has helped many people get the testing and treatment they need to live long and healthy lives.

But what is the p24 antigen, and how has it changed the testing process?

Let’s get into it.

If you’re worried about HIV, get tested at a sexual health clinic near you.

What is an antigen?

Antigens are protein structures on the outside of organisms foreign to your body, like viruses and bacteria. Each antigen is unique to the pathogen that creates it. 

When your immune system’s B cells come into contact with an antigen, they recognise it and release antibodies into your bloodstream. These antibodies latch onto the pathogen’s antigens and tag them for attack by your other immune cells.

What is the p24 antigen?

One distinctive antigen released by the HIV virus has been named p24. It’s a structural protein that makes up most of the viral core (or capsid).

When someone is newly infected with HIV, high levels of the p24 antigen can be found in the blood serum before seroconversion takes place. (“Seroconversion” refers to the period when HIV antibodies first become detectable in the body.)

However, this doesn’t last long. Because the body produces antibodies for the p24 antigen, the antigen will become undetectable after a certain period. P24 antigen assays, therefore, aren’t a reliable test for HIV in the later stages of the infection (usually up to 50 days after exposure).

Why do you test for the p24 antigen?

Because the p24 antigen appears in blood serum before seroconversion, identifying the antigen within a blood sample is a reliable way of diagnosing an acute, or early, HIV infection.

This is usually done with a 4th generation HIV test, which tests for the p24 antigen and antibodies the immune system creates to fight the virus.

Because the p24 antigen appears in the bloodstream at a specific time, an antigen assay is ideal for detecting the infection in its early stages, usually 10-20 days after the initial infection. This is critical for early detection of the infection so that interventions like antiretroviral therapy (ART) can take place.

Final thoughts

HIV doesn’t have to up-end your life completely. It’s possible to live a long and healthy life while being HIV-positive.

If you’d like to know your HIV status, click the button below to start an online booking, or call the number at the top of this page to speak to one of our sexual health advisors.

Better2Know makes HIV testing a breeze. Get tested at a sexual health clinic near you.