There is lots of comment about the supposed sudden disappearance of p24 antigen at 28 days. This does not happen and the p24 element will remain elevated for at least 8 to 10 weeks and if you look hard enough it is present through the remainder of the illness.
P24 is a by-product of viral replication. As viral replication increases with initial infection then p24 is produced in huge amounts. As viral replication slows as your body hopefully attempts to regain control p24 will drop. However, the majority of people infected with HIV will develop antibdoies detectable by third generation tests at 4 weeks and over 99% by 6 weeks. 1st and 2nd generation HIV tests - still in use in certain parts of the world (including parts of the US until 2005) were poor, gave variable results and were unreliable.