HIV can be blocked because HIV mainly invades the immune system of the body, and when the virus enters the body, it will reach the local lymph nodes and start replicating within 24-48 hours, so it can be blocked by taking antiviral drugs during this time. Generally, after a high-risk behavior, you should take the blocking drugs within 2h as much as possible to achieve the maximum blocking effect. If it is not possible to do so within 2h, you should take the blocking drug within 48h, which still has the effect of blocking. If for any reason you cannot get the drug within 48h, you should also take the blocking drug within 72h. By taking a complete blocking cycle, there is still hope that the blocking will be successful.