A method is provided for removing red blood cells from a suspension comprising red blood cells, white blood cells, platelets and plasma using a spinning membrane separator. The method comprises: a) flowing whole blood into the gap of the spinning membrane separator; b) collecting red blood cells and white blood cells in the gap and passing plasma and platelets through the membrane; c) introducing a first quantity of lysing buffer into the gap; d) incubating the red blood cells, white blood cells and lysing buffer in the gap for a period of time to cause a lysis reaction with the red blood cells; e) introducing a second quantity of lysing buffer into the gap to displace the first quantity of lysing buffer and a first quantity of red blood cell debris out of the gap; f) introducing a first quantity of wash buffer into the gap to quench the lysis reaction and displace the second quantity of lysing buffer and a second quantity of red blood cell debris out of the gap; and g) introducing a second quantity of wash buffer into the gap to flow washed white blood cells out of the housing.