A filter for purifying blood (1, 100), comprising a container body (2, 102) in which there are: —an intake port (3, 103) for an incoming blood stream (4, 104), —a first discharge port (5, 105) for an outgoing blood stream (6, 106), and —a second discharge port (7, 107) for a purified plasma stream (8, 108); the container body (2, 102) comprises first filtering means (9, 109) adapted to separate a stream of plasma to be purified (10, 100) from the incoming blood stream (4, 104), and second filtering means (11, 111) adapted to purify the stream of plasma to be purified (10, 110) to obtain the purified plasma stream (8, 108).