Patent Publication Number: US-8992402-B2

Title: System for blood separation with side-tapped separation chamber

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority of U.S. Provisional Application Ser. No. 61/496,164, filed on Jun. 13, 2011. The disclosure of the above-identified application is hereby incorporated by reference in its entirety as if set forth herein in full for all that it teaches and for all purposes. 
    
    
     BACKGROUND OF INVENTION 
     Blood collection and processing play important roles in the worldwide health care system. In conventional large scale blood collection, blood is removed from a donor or patient, separated into its various blood components via centrifugation, filtration, or elutriation and stored in sterile containers for future infusion into a patient for therapeutic use. The separated blood components typically include fractions comprising red blood cells, white blood cells, platelets, and plasma. Separation of blood into its components can be performed continuously during collection or can be performed subsequent to collection in batches, particularly with respect to the processing of whole blood samples. Separation of blood into its various components under highly sterile conditions is critical to many therapeutic applications. 
     Recently, apheresis blood collection techniques have been adopted in many large scale blood collection centers wherein a selected component of blood is collected and the balance of the blood is returned to the donor during collection. In apheresis, blood is removed from a donor and immediately separated into its components by on-line blood processing methods. Typically, on-line blood processing is provided by density centrifugation, filtration, or diffusion-based separation techniques. One or more of the separated blood components are collected and stored in sterile containers, while the remaining blood components are directly re-circulated to the donor. An advantage of this method is that it allows more frequent donation from an individual donor because only a selected blood component is collected and purified. For example, a donor undergoing plateletpheresis, whereby platelets are collected and the non-platelet blood components are returned to the donor, may donate blood as often as once every fourteen days. 
     Apheresis blood processing also plays an important role in a large number of therapeutic procedures. In these methods, blood is withdrawn from a patient undergoing therapy, separated, and a selected fraction is collected while the remainder is returned to the patient. For example, a patient may undergo leukapheresis prior to radiation therapy, whereby the white blood cell component of his blood is separated, collected and stored to avoid exposure to radiation. 
     Both conventional blood collection and apheresis systems typically employ differential centrifugation methods for separating blood into its various blood components. In differential centrifugation, blood is circulated through a sterile blood processing vessel which is rotated at high rotational speeds about a central rotation axis. Rotation of the blood processing vessel creates a centrifugal force directed along rotating axes of separation oriented perpendicular to the central rotation axis of the centrifuge. The centrifugal force generated upon rotation separates particles suspended in the blood sample into discrete fractions having different densities. Specifically, a blood sample separates into discrete phases corresponding to a higher density fraction comprising red blood cells and a lower density fraction comprising plasma. In addition, an intermediate density fraction comprising platelets and leukocytes forms an interface layer between the red blood cells and the plasma. Descriptions of blood centrifugation devices are provided in U.S. Pat. No. 5,653,887 and U.S. Pat. No. 7,033,512. 
     To achieve continuous, high throughput blood separation, extraction or collect ports are provided in most blood processing vessels. Extraction ports are capable of withdrawing material from the separation chamber at adjustable flow rates and, typically, are disposed at selected positions along the separation axis corresponding to discrete blood components. To ensure the extracted fluid exiting a selected extraction port is substantially limited to a single phase, however, the phase boundaries between the separated blood components must be positioned along the separation axis such that an extraction port contacts a single phase. For example, if the fraction containing white blood cells resides too close to the extraction port corresponding to platelet enriched plasma, white blood cells may enter the platelet enriched plasma stream exiting the blood processing vessel, thereby degrading the extent of separation achieved during blood processing. Although conventional blood processing via density centrifugation is capable of efficient separation of individual blood components, the purities of individual components obtained using this method is often not optimal for use in many therapeutic applications. 
     As a result of the inability to achieve optimal purity levels using centrifugation separation alone, a number of complementary separation techniques based on filtration, elutriation in a cell separation chamber and affinity-based techniques have been developed to achieve the optimal purities needed for use of blood components as therapeutic agents. These techniques, however, often reduce the overall yield realized and may reduce the therapeutic efficacy of the blood components collected. Exemplary methods and devices of blood processing via filtration, elutriation and affinity based methods are described in U.S. Pat. No. 6,334,842. 
     A centrifugal blood component separation apparatus has been described in commonly assigned U.S. Pat. No. 7,605,388, for instance. As described in U.S. Pat. No. 7,605,388, an optical cell may be configured such that white blood cells can be extracted through a first extraction port, plasma and/or platelets can be extracted through second extraction port, and red blood cells can be extracted through third extraction port. As also mentioned in U.S. Pat. No. 7,605,388 (but not shown), optical cells of a blood separation vessel can include one or more dams positioned proximate to the extraction ports to facilitate selective extraction of separated blood components having reduced impurities arising from adjacent components. The use of dams in blood processing via density centrifugation is known in the art and described in U.S. Pat. Nos. 6,053,856; 6,334,842 and 6,514,189. 
     SUMMARY OF THE INVENTION 
     This invention provides methods, devices and device components for improving the processing of fluids comprising fluid components, such as blood, components of blood and fluids derived from blood. Methods, devices and device components of the present invention are capable of monitoring and controlling separation of blood into discrete components and subsequent collection of selected components. In particular, it has been found that white blood cells may be extracted from the bottom of a fluidized bed leuko-reduction chamber (or cell separation chamber) while plasma and platelets are removed from the top of the separation chamber. The system and method may enable collection of white blood cells with fewer platelets. 
     A function of the centrifuge blood processing system described herein may be the collection of white blood cells or other selected blood components such as mesenchymal stem cells. In a preferred embodiment, certain functions of the centrifugal blood separator are controlled by an optical monitoring system. A cell separation chamber, adapted to be mounted on a rotor of the centrifuge blood processing system, comprises an inlet for receiving plasma, platelets and white blood cells, or “buffy coat”, an outlet for ejecting plasma and platelets from the separation chamber, and a side tap outlet for ejecting white blood cells and plasma from the separation chamber. Red blood cells or plasma may be collected or returned to a donor. White cells or other components such as mesenchymal stem cells and plasma may be collected for therapeutic purposes. 
     According to an embodiment, an optical cell of a circumferential blood processing vessel comprises at least a buffy coat extraction port and a red blood cell extraction port. White cells collect at the buffy coat extraction port. This configuration allows white cell-containing buffy coat to be withdrawn from the blood processing vessel through the buffy coat extraction port for further separation in the fluidized-bed filtration chamber or cell separation chamber. The cell separation chamber has a generally conical shape, with a buffy coat inlet at the apex of the cone and adapted to be mounted with the buffy coat inlet radially outwardly on the centrifuge rotor. A plasma outlet is centrally located in the base of the cone and is adapted to be mounted radially inwardly on the centrifuge rotor. The base may also have a slight conical shape to conduct platelets and plasma to the plasma outlet. The inlet comprises a pipe or tube extending into the interior of the separation chamber such that a circumferential well is formed between the pipe and an interior conical wall of the separation chamber. A side tap white blood cell extraction port penetrates the conical wall into the circumferential well. White blood cells (“WBC”) fall into the well and white blood cells and plasma are withdrawn from the separation chamber through the side tap extraction port for collection. 
     A function of the apparatus and method described herein may be to reduce the loss of white blood cell product or selected cell components such as mesenchymal stem cells that can occur during periodic flushing of white blood cells through the platelet outlet, as in conventional separation chamber. 
     Further the apparatus may provide more continuous steady flow through a cell separation chamber, thereby providing a greater volume of blood components processed per unit time. Another feature may be to produce a collected white blood cell (or other selected cell types) product having fewer platelets than conventional collection methods, and thus improved purity. 
     According to an embodiment, collection flow rates out of the separation chamber may be reduced and the volume of WBC-containing fluid extracted from the separation chamber may be reduced. A low WBC extraction volume may be achieved with a cycled extraction rate through the side-tap port that may be triggered by detection of a saturated separation chamber by the optical sensor. Total flow through the separation chamber may be kept constant. 
     An embodiment may have a cell separation chamber for cell collection that is not limited by insufficient available plasma for flushing the selected cells through the platelet outlet, as in conventional separation chamber. 
     For donors whose blood has a high hematocrit, it has sometimes been difficult or impossible to reduce the RPM of the centrifuge rotor (and thereby reduce the gravitational field of the centrifuge) sufficiently to allow complete flushing of WBC out of the platelet outlet of a conventional separation chamber. An embodiment may not require a reduction in the centrifuge gravitational field when white blood cells are removed from the separation chamber. 
     These and other features and advantages of the invention will be apparent from the following description, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of one embodiment of an apheresis system, which can be used in or with the present invention. 
         FIG. 2  illustrates a tubing and bag set including an extracorporeal tubing circuit, a cassette assembly, collection bag assembly, a blood processing vessel, and a cell separation chamber for use in or with the system of  FIG. 1 . 
         FIG. 3  is a perspective view of a blood processing vessel and the cell separation chamber. 
         FIG. 4  is a plan view of the cell separation chamber of  FIG. 3 . 
         FIG. 5  is a cross-sectional view of the cell separation chamber of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     To describe the present invention, reference will now be made to the accompanying drawings. The present invention may be used with a blood processing apparatus such as a SPECTRA OPTIA® blood component centrifuge manufactured by CaridianBCT, Inc, or a TRIMA® or TRIMA ACCEL® blood component centrifuge also manufactured by CaridianBCT, Inc. The invention may also be used with other blood component centrifuges. The above-named centrifuges incorporate a one-omega/two-omega seal-less tubing connection as disclosed in U.S. Pat. No. 4,425,112 to provide a continuous flow of blood to and from the rotor of an operating centrifuge without requiring a rotating seal. 
     The an embodiment of the invention may comprise an improved leuko-reduction or cell separation chamber for removal of white blood cells (“WBC”) or other selected types of cells such as mesenchymal stem cells from blood components. A related separation chamber is described in commonly-assigned U.S. application Ser. No. 12/209,793. 
     It is desirable for a separation chamber to separate greater than 99.99% of entrained WBC from platelet or plasma products obtained by centrifugal apheresis, which is an extremely high value. The process for this separation is based on the phenomenon of particle sedimentation in a fluid. The separated WBC consist of about 95% mononuclear cells (which are about 90% leukocytes and 10% monocytes) and about 5% granulocytes. To accommodate the apheresis collection process, the separation chamber may function in an automatic mode as a continuous-feed process. This requires an overflowing saturated bed of platelets above a bed of mononuclear cells, which continuously accumulate during the collection. The saturated bed requirement operates in the dense-phase flow regime, which is characterized by high cell density. After a quantity or bolus of white blood cells are collected in the separation chamber, the WBC are removed from the chamber for collection. In devices with conventional separation chambers, this may be accomplished by reducing the rotational speed of the centrifuge and increasing the flow rate of plasma through the separation chamber, thus pushing the WBC bolus out of the outlet port. An additional line or tube, however, is connected to a side wall of the separation chamber through a side tap, near an inlet to the chamber. The WBC are drawn out of the separation chamber through the side tap, which is “down hill” with respect to the gravitational field created by the centrifuge apparatus. There is little or no need to change the speed of the centrifuge, nor is an increased inlet flow of plasma needed to flush the collected WBC through the outlet. However, briefly pausing a pump in the collect line will pack WBC in the separation chamber, allowing the cells to ultimately be removed from the separation chamber in a smaller volume. A relatively small amount of additional plasma may be allowed to flow back into the chamber from the outlet, or to flow in from the inlet to displace the WBC and fluid being withdrawn through the side tap. Preferably, the separation chamber is oriented such that the side tap is on the trailing side of the separation chamber when the chamber is mounted on the rotor. That is, as the rotor turns, the side tap is on the side of the separation chamber that passes an observer last. A protruding inlet port may also be provided coupled to the inlet of the separation chamber and adjacent the side tap outlet. 
     The protruding inlet port is a tube that transfers the entering flow past a critical area where the wall of the chamber forms the apex of a cone opening into the body of the chamber and past a side tap outlet for withdrawing WBC from the chamber. The protruding port may eliminate a flow path along the wall that is caused by Coriolis acceleration. Coriolis acceleration pushes fluid entering the chamber towards the leading chamber wall. This entering fluid contains high concentrations of WBC. If the fluid is pushed against the wall, rather than remaining generally in the center of the chamber, the fluid tends to flow up the wall, circumventing the bed of white blood cells and platelets that captures WBC in the chamber by sedimentation forces. In addition, the protruding inlet port blocks direct entry into the side tap outlet, thus compelling the WBC to enter the area of the fluidized bed within the separation chamber. 
     A preferred blood apheresis system  2  for use with the present invention is schematically illustrated in  FIG. 1 . System  2  provides for a continuous blood component separation process. Generally, whole blood is withdrawn from a donor and is substantially continuously provided to a blood component separation device  6  where the blood is separated into various components and at least one of these blood components is collected from the device  6 . One or more of the separated blood components may be either collected for subsequent use or returned to the donor. 
     In the blood apheresis system  2 , blood is withdrawn from the donor and directed through a bag and tubing set  8 , which includes an extracorporeal tubing circuit  10 , and a blood processing vessel  12 , which together define a closed, sterile and disposable system. The set  8  is adapted to be mounted in the blood component separation device  6 . The separation device  6  includes a pump/valve/sensor assembly  14 , which interfaces with the extracorporeal tubing circuit  10 , and a centrifuge assembly  16 , which interfaces with the blood processing vessel  12 . 
     The centrifuge assembly  16  may include a channel  18  in a rotatable rotor assembly  20 , which provides the centrifugal forces required to separate blood into its various blood component types by centrifugation. The blood processing vessel  12  may then be fitted within the channel  18 . Blood can flow substantially continuously from the donor, through the extracorporeal tubing circuit  10 , and into the rotating blood processing vessel  12 . Within the blood processing vessel  12 , blood may be separated into various blood component types and at least one of these blood component types (e.g., white blood cells, platelets, plasma, or red blood cells) may be removed from the blood processing vessel  12 . Blood components that are not being retained for collection or for therapeutic treatment (e.g., platelets and/or plasma) are also removed from the blood processing vessel  12  and returned to the donor via the extracorporeal tubing circuit  10 . Various alternative apheresis systems (not shown) may also make use of the present invention, including batch processing systems (non-continuous inflow of whole blood and/or non-continuous outflow of separated blood components) or smaller scale batch or continuous RBC/plasma separation systems, whether or not blood components may be returned to the donor. 
     Operation of the blood component separation device  6  is controlled by one or more processors included therein, and may advantageously comprise a plurality of embedded computer processors to accommodate interface with PC user facilities (e.g., CD ROM, modem, audio, networking and other capabilities). In order to assist the operator of the apheresis system  2  with various aspects of its operation, the blood component separation device  6  includes a graphical interface  22  with an interactive touch screen. 
     An extracorporeal tubing circuit  10 , shown in  FIG. 2 , may include a cassette  26  and a number of tubing/collection assemblies  28 ,  32 ,  34 ,  36 ,  38  and  40 . A blood removal-return tubing assembly  28  provides a needle interface for withdrawing blood from a donor to the remainder of the tubing circuit  10  and for returning blood components and other fluids to the donor. A single needle configuration is shown, but a double needle interface may also be used. Three lines  42 ,  44 ,  46  are provided in blood removal-return tubing assembly  28  for removal of blood from the donor. A cassette  26  is connected between the tubing assembly  28 , which connects to the donor, and blood inlet/blood component tubing line sub-assembly  32 , which provides the interface between cassette  26  and blood processing vessel  12 . The cassette  26  orients tubing segments in predetermined spaced relationships within the cassette  26  for ultimate engagement with valve members on apheresis device  6 . Such valves will, when activated, control flow through loops and tubing. 
     The tubing line sub-assembly  32  comprises five lines  60 ,  62 ,  64 ,  65 , and  66 , shown in  FIG. 2 , for transport of blood and components to and from the processing vessel  12 . The five lines are encased in a sheath  33  that allows the one omega-two omega motion described in U.S. Pat. No. 4,425,112. An anticoagulant tubing assembly  40 , a vent bag  34 , a plasma collection assembly  36 , and a white blood cell collection bag  38  are also interconnected with cassette  26 . Optionally, a red blood cell collection assembly might also be provided through an auxiliary line  96 , as is known in the art. The extracorporeal tubing circuit  10  and blood processing vessel  12  are pre-connected to form a closed, sterilized, disposable assembly for a single use. 
     When the tubing circuit  10  has been mounted on the blood component separation device  6 , saline solution (not shown) primes the tubing circuit through a line  54  and filter  56  (see  FIG. 2 ). Saline flows through an internal passageway in the cassette  26  and through the line  44  to the distal end of the blood removal-return assembly  28 . Saline can then flow up a blood withdrawal line  42  into the other tubes and passageways of the circuit  10  in preparation for blood processing. A supply or bag (not shown) of anticoagulant can then be connected to a distal end of the anticoagulant tubing assembly  40  in place of a saline supply. Anticoagulant solution flows past the filter  56  and a first pump loop  58  through the anticoagulant line  44  to the distal end of the blood removal assembly. The pump loop  58  and other pump loops described herein couple with peristaltic pumps on the blood processing device  6  in a known manner. The device  6  controls the direction and rate of flow of the fluids described herein by controlling the speed and direction of the peristaltic pumps and the position of various valves. 
     The blood removal line  42  conducts blood into the cassette  26 , where the blood passes a first pressure sensor  80  and a second pump loop  82 . A second pressure sensor  84 , between second pump loop  82  with its associated pump and blood inflow line  60  to the blood processing vessel  12 , senses the fluid pressure effective at an inlet to the blood processing vessel  12 . Emanating from blood processing vessel  12  is an RBC outlet tubing line  62  of the blood inlet/blood component tubing assembly  32 . The outlet tubing line  62  connects to an external loop  86  to a return reservoir  88 . The return reservoir  88  contacts sensors on the device  6  that detect low and high fluid levels. The device  6  keeps the fluid in the reservoir between these two levels by controlling flow out of the reservoir past a return pump loop  90  and a return pressure sensor  92 . As the fluid level in the reservoir  88  is constantly rising and falling, a vent bag  34  connects to the reservoir  88  through a vent tube  94 . Air can flow between the reservoir  88  and the vent bag  34  in a sterile manner. Fluid flows into a return tube  46  in the blood removal-return assembly  28 . The blood removal-return assembly  28  also comprises the line  44  for priming or anti-coagulant as described above. If desired, red blood cells could be withdrawn through auxiliary line  96  and collected in a collection bag (not shown). Alternatively, a bag containing replacement fluid (not shown) is connect to a spike or Luer connector  98  on the replacement line, allowing replacement fluid to pass through the return loop  86  into the reservoir  88 . Blood components and replacement fluid are then returned to the donor. In the present embodiment, replacement line  96  is connected to return loop  86  through a junction  99  and manual closures or clamps are provided to direct the flow of red blood cells and replacement fluid. Equivalently, it is known to couple the red blood cell line  62  to a peristaltic pump and to provide an automatic valve to select blood flow paths, as shown, for instance in U.S. patent application Ser. No. 12/959,987. 
     Plasma may also be collected from the blood processing vessel  12  into plasma bag  36 . When desired, plasma is withdrawn from the blood processing vessel  12  through plasma line  66  to a pump loop  100 . A valve  101  diverts the plasma either into a collect tube  102  to the plasma bag  36 , or into connecting loop or line  104  to the reservoir  88 . Excess plasma in the reservoir  88  is returned to the donor in the same way as red blood cells, as described above. 
     White blood cells and platelets flow out of the blood processing vessel  12  through a cell line  68  into a cell separation chamber  114 , which is further described below. The contents of the separation chamber flow out of the separation chamber either through a primary outlet line  64  at the primary outlet  116  (see  FIG. 4 ) or through a secondary outlet line  65  at a secondary or side tap outlet  119  near the inlet  118  of the separation chamber, as will be discussed below. The primary and secondary outlet lines  64 ,  65  pass through the tubing line sub-assembly  32  and sheath  33  to the cassette  26 . A valve  123  selects between the two outlet lines  64 ,  65 , closing one line and opening the other, thereby allowing fluid to be withdrawn from two different locations in the separation chamber  114 . In the cassette  26 , the fluid from the selected outlet line passes a red-green photo sensor  106 , which may be used to control periodic flushing of white blood cells out of the cell separation chamber  114  into the collect bag  38 . The selected cells flow through a pump loop or common line  108 , which engages a peristaltic pump on the separation device  6 . The pump loop  108  connects to a valved passageway in the cassette  26 . The blood processing device  6  can control a valve  121  to direct white blood cells or other selected cells either into a collect tube  110  and thence into the collect bag  38 , or into a connection loop or line  112  and thence into the reservoir  88 . For platelet collection, excess white blood cells in the reservoir  88  may be returned to the donor in the same way as red blood cells and plasma, as described above. Alternatively, for mesenchymal stem cell (MNC) collection, wherein platelets are usually returned to the donor, the MNC are withdrawn through the side tap outlet  119  into the collect tube  110  for storage in the collect bag  38 . 
     The process of extracting cells from the separation chamber has been described using a single pump  128  (see  FIG. 3 ) connected to loop  108  for both line  64  from the to primary outlet  116  and for the secondary outlet line  65  from the side tap outlet  119 . Clearly, an additional loop and pump could also be provided such that each of the outlet lines  64 ,  65  would be provided with a dedicated loop and pump. The valve  123  would not be required to selectively couple the two lines  64 ,  65  to a shared loop and pump. Such variations are within the scope of one skilled in the art, without departing from the teachings of the present invention. 
     During a blood removal, whole blood will be passed from a donor into tubing line  42  of blood removal tubing assembly  28 . The blood is pumped by the device  6  via pump loop  82 , to the blood processing vessel  12  via the cassette  26  and line  60  of the blood inlet/blood component tubing assembly  32 . Separation processing then occurs on a substantially continuous basis in the blood processing vessel  12 , i.e., blood flows substantially continuously therein, is continuously separated and flows as separated components therefrom. After separation processing in vessel  12 , uncollected blood components are transferred from the processing vessel  12  to and through cassette  26  and into reservoir  88  of cassette  26 , which is filled up to a predetermined level. The blood component separation device  6  may initiate a blood return submode wherein components may be returned to the donor through return line  46 . The cycle between blood removal and blood return submodes will continue until a predetermined amount of blood components have been harvested. In an alternative double needle scheme, as is known in the art, blood may be removed from the donor and returned to a donor through two separate needles. See, for example, US Patent application 2010/160137. 
     A bracket (not shown) is provided on a top surface of the centrifuge assembly  16 . The bracket releasably holds the cell separation chamber  114  on the centrifuge assembly  16  so that an outlet  116  of the cell separation chamber  114  is positioned closer to the axis of rotation than an inlet  118  of the chamber  114 . The bracket orients the chamber  114  on the centrifuge assembly  16  with a longitudinal axis of the cell separation chamber  114  in a plane transverse to the rotor&#39;s axis of rotation. In addition, the bracket is arranged to hold the cell separation chamber  114  on the centrifuge assembly  16  with the cell separation chamber outlet  116  facing the axis of rotation. Although the chamber  114  is preferably on a top surface of the centrifuge assembly  16 , the chamber  114  could also be secured to the centrifuge assembly  16  at alternate locations, such as beneath the top surface of the centrifuge assembly  16 . 
       FIG. 3  schematically illustrates the blood processing vessel  12  and cell separation chamber  114 . The blood processing vessel  12  has a generally annular flow path and includes an inlet portion  120  and an outlet portion  122 . The inflow tube  60  connects to the inlet portion  120  for conveying a fluid to be separated, such as whole blood, into the blood processing vessel  12 . During a separation procedure, substances entering the inlet portion  120  flow around the vessel  12  and stratify according to differences in density in response to rotation of the centrifuge assembly  16 . The outlet portion  122  includes outlets for the RBC line  62 , the plasma line  66 , and buffy coat or white blood cell line  68  for removing separated substances from the blood processing vessel  12 . Each of the components separated in the vessel  12  is collected and removed in only one area of the vessel  12 , namely the outlet portion  122 . 
     The outlet of the line  68  is connected to the cell separation chamber inlet  118  to pass intermediate density components, including white blood cells or mesenchymal stem cells (MNC), into the cell separation chamber  114 . Components initially separated in the blood processing vessel  12  are further separated in the cell separation chamber  114 . For example, white blood cells could be separated from plasma and platelets in the cell separation chamber  114 . This further separation takes place by forming a saturated fluidized bed of particles in the cell separation chamber  114 . Plasma and platelets would flow out of the cell separation chamber  114  while white blood cells were retained in the chamber. Similarly, granulocytes could be separated from red blood cells in like manner. 
     As schematically shown in  FIG. 3 , a plurality of pumps  124 ,  126 , and  128  are provided for adding and removing substances to and from the blood processing vessel  12  and cell separation chamber  114 . An inflow pump  124  is coupled to the inflow line  60  at pump loop  82  ( FIG. 2 ) to supply the substance to be separated, such as whole blood, to the inlet portion  120 . In addition, a first collection pump  126  is coupled at loop  100  to the plasma line  66 . A second collection pump  128  is coupled to the collection line  64  and the cell collection line  65  at loop  108 . The second collection pump  128  draws liquid and particles either from the cell separation chamber outlet  116  or from the side tap outlet  119  and causes liquid and particles to enter the cell separation chamber  114  via the cell separation chamber inlet  118 . In the present embodiment, plasma and platelets are usually withdrawn from the outlet  116  of the cell separation chamber  114  through line  64 . In the prior art, collected white blood cells or MNC or other components would be flushed from the chamber  114  through the first cell collection line  64  by either increasing the fluid flow through the chamber  114  or by slowing the rotor or both. On the other hand, in the illustrated embodiment, the second cell collection line  65  connects to the cell collection chamber  114  near the inlet  118  at the side tap connection  119 . A valve  123  on the blood component separation device  6  selectively closes the collection lines  64 ,  65 . The first and second collection lines  64 ,  65  are joined into a common tube past the valve  123  and form loop  108 . Thus, the peristaltic pump  128 , which is coupled to loop  108 , can draw fluid either from the outlet  116  or the side tap outlet  119 , depending on the valve  123 . Alternatively, as described above, a dedicated pump and loop could be provided for each of the collect lines  64 ,  64  without departing from the teachings of the present invention. 
     During the formation of the fluidized bed of cells in the chamber  114 , platelet rich plasma (PRP) or platelets would ordinarily be drawn from the outlet  116  through tube  64 . During expression of collected cells (e.g., MNC), the collected cells would be drawn through the side tap  119  and second collection line  65 . Since the collected cells would be relatively heavier than plasma, they would tend to fall towards the side tap  119  and could more easily be withdrawn from the chamber  114 . Beyond pump  128 , loop  108  again divides into two lines  110 ,  112 . A valve  121  on the device  6  selectively opens and closes the lines. Line  112  is coupled to the reservoir  88  and ordinarily returns PRP to the donor. Line  110  is coupled to a collect bag  38  and allows the collected cells to flow into the collect bag  38 . 
     The first collection pump  126 , which is coupled to loop  100 , removes primarily low-density substances such as plasma directly from the blood processing vessel  12  via the plasma line  66 . The plasma could either be collected in plasma bag  36  through line  102 , or returned to the donor through connecting loop or line  104  and the reservoir  88 . Valve  101  selectively opens and closes the lines  102 ,  104  to direct the flow of plasma either to the bag  36  or to the reservoir  88 . 
     The pumps  124 ,  126 , and  128  are peristaltic pumps, which prevent significant damage to blood components. The pumps  124 ,  126 , and  128  control the flow rate of substances flowing to and from the blood processing vessel  12  and the cell separation chamber  114 . A saturated fluidized bed of particles is maintained within the cell separation chamber  114  to cause other particles to be retained in the cell separation chamber  114 . 
     Blood within the processing vessel  12  is subjected to centrifugal force causing components of the blood components to separate. The components of whole blood stratify in order of decreasing density as follows: (1) red blood cells, (2) white blood cells, (3) platelets, and (4) plasma. The controller regulates the rotational speed of the centrifuge channel assembly  16  to ensure that this particle stratification takes place. A layer of red blood cells (high density components) forms along the outer wall of the processing vessel  12  and a layer of plasma (lower density components) forms along the inner wall of the processing vessel  12 . Between these two layers, the intermediate density platelets and white blood cells (intermediate density components) form a buffy coat layer. Preferably, the separation is observed in two dimensions by a camera and controlled as described in U.S. Pat. No. 7,422,693, which is incorporated herein by reference. 
     The cell separation chamber is shown in detail in  FIG. 4  and  FIG. 5 .  FIG. 5  is a cross section of  FIG. 4  taken in the plane of the illustration. The cell separation chamber  114  may be constructed in two pieces, a main body  200  and a cap  202 , both being symmetrical around an axis  204 . The main body  200  has an inlet  118  comprising a through bore  206  and a concentric stopped bore  208 . The diameter of the through bore  206  corresponds to the inside diameter of the cell line  68 , while the diameter of the stopped bore  208  corresponds to the outside diameter of the line  68 , so that the cell line  68  can be seated in the stopped bore  208  and a fluid passageway of constant diameter can be formed between the line  68  and the through bore  206 . The through bore  206  opens into a frustro-conical segment  210 . A wall  212  of the frustro-conical segment  210  may comprise a plurality of steps  214  which generally taper away from the axis  204 . Straight or curved walls may also be used in the frustro-conical segment  210 . The through bore  206  rises into the frustro-conical segment  210  through a protruding inlet  216 . A mouth  218  of the protruding inlet  216  opens into the frustro-conical segment  210  spaced away from the wall  212 , thereby forming a circumferential well  220  between the wall and the protruding inlet. A stream of fluid leaving the protruding inlet and entering the chamber is insulated from the effects of the wall  220  by a relatively static fluid layer. The stream is therefore less likely to adopt a flow path along the wall, under the influence of Coriolis forces, but rather will remain in the center of the chamber, allowing more uniform mixing of cells and other particles within the chamber. An inner surface  219  of the protruding inlet flares slightly outwardly towards the mouth  218  of the protruding inlet  216 . This reduces the flow velocity of fluid passing through the protruding inlet and lessens Coriolis effects as the fluid enters the chamber. 
     In the illustrated embodiment, the main body  200  of the cell separation chamber  114  further comprises a circumferential flange  244 , which is supported in the holder. The cap  202  comprises a rim  246  that fits against the flange  244 . An interlocking groove and ridge (not shown) may be provided between the rim  246  and flange  244  for sealing, if desired. The cap  202  and main body  200  may be joined by ultrasonic welding or other suitable techniques as known in the art. The cap opens into an abrupt frustro-conical segment  248 . The abrupt segment  248  tapers towards the axis  204 . The abrupt segment  248  funnels blood components into the outlet  116  without excessive turbulence or damage to the blood components. The outlet  116  comprises a through bore  250  and a concentric stopped bore  252 . The diameter of the through bore  250  corresponds to the inside diameter of the cell line  64 , while the diameter of the stopped bore  252  corresponds to the outside diameter of the cell line  64 , so that the line  64  can be seated in the stopped bore  252  and a fluid passageway of constant diameter can be formed between the line  64  and the through bore  250 . The through bore  250  opens into the frustro-conical segment  248 . 
     As described above, the separation chamber  114  further comprises a side tap outlet  119 . The outlet  119  also comprises a through bore  228  and a concentric stopped bore  230 . The diameter of the through bore  228  corresponds to the inside diameter of the cell line  65 , while the diameter of the stopped bore  230  corresponds to the outside diameter of the cell line  65 , so that the line  65  can be seated in the stopped bore  230  and a fluid passageway of constant diameter can be formed between the line  65  and the through bore  228 . 
     The separation chamber  114  functions in an automatic mode as a continuous-feed process. An overflowing saturated bed of platelets forms above a bed of mononuclear cells, which continuously accumulate during the collection. The saturated bed requirement operates in the dense-phase flow regime, which is characterized by high cell density. After a quantity or bolus of white blood cells or other selected cells is collected in the separation chamber, the cells are removed from the chamber for collection. The selected cells may be drawn out of the separation chamber  114  through the side tap outlet  119 , which is “down hill” with respect to the gravitational field created by the centrifuge apparatus. There is little or no need to change the speed of the centrifuge, nor is an increased inlet flow of plasma needed to flush the collected cells through the outlet  116 . With the line  64  closed and the line  65  open, the collected cells are drawn out of the separation chamber for collection through the side tap outlet  119 . A relatively small amount of additional plasma may be allowed to flow back into the chamber  114  from the outlet  116 , or to flow in from the inlet  118  to displace the cells and fluid being withdrawn through the side tap outlet  119 . 
     Although the inventive device and method have been described in terms of filtering white blood cells, this description is not to be construed as a limitation on the scope of the invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the structure and methodology of the present invention without departing from the scope or spirit of the invention. Rather, the invention is intended to cover modifications and variations provided they come within the scope of the following claims and their equivalents.