Method for processing a biological fluid including leukocyte removal in an extracorporeal circuit

A method for removing undesirable material including leukocytes from a biological fluid in an extracorporeal circuit comprising:

• passing a leukocyte containing biological fluid from a patient into a container;
• passing a portion of biological fluid from the container through a recirculation loop including a filter assembly for removing leukocytes, wherein passing the portion of biological fluid through the recirculation loop provides a leukocyte depleted portion of biological fluid, and includes passing the portion of leukocyte depleted biological fluid into the container;
• passing additional leukocyte containing biological fluid from a patient into the container so that the additional fluid is mixed with leukocyte depleted biological fluid in the container to provide a mixture including leukocyte depleted and non-leukocyte depleted biological fluid;
• passing the mixture of biological fluid from the container, and passing a portion of the mixture to the patient while passing another portion of the mixture through the recirculation loop and the filter assembly and into the container.