What is the window phase of an infection? How is this concept important for the test of HIV infection in blood banks?

The primary immune response of the body facing any infection is not instant. The window phase is the period from the infection unless the formation of detectable specific antibodies against the infective agent. In this period, immunochemical tests that not directly search infections, like the ELISA and the Western-blot for HIV, give a negative result even if the person is in fact infected by the agent.