An HIV antigen (also known as the p24 antigen) is a protein the virus introduces into the immune system of the infected person, triggering it to start making antibodies.
This structural protein makes up most of the HIV viral core, or 'capsid,' and its present in high levels in the blood serum of recently infected individuals during the short period between infection and seroconversion.
Certain tests can screen for HIV antigens and detect primary infection much more reliably than only tests that screen for antibodies.