Patent Publication Number: US-2005137517-A1

Title: Processing systems and methods for providing leukocyte-reduced blood components conditioned for pathogen inactivation

Description:
RELATED APPLICATIONS  
      This application claims the benefit of the priority date of copending U.S. patent application Ser. No. 10/008,361, filed Dec. 5, 2001 and entitled “Manual Processing Systems and Methods for Providing Blood Components Conditioned for Pathogen Inactivation.” 
    
    
     FIELD OF THE INVENTION  
      The invention generally relates to the processing of whole blood and its components for storage, fractionation, and transfusion.  
     BACKGROUND OF THE INVENTION  
      The clinically proven components of whole blood include, e.g., red blood cells, which can be used to treat chronic anemia; plasma, which can be used as a blood volume expander or which can be fractionated to obtain Clotting Factor VIII-rich cryoprecipitate for treatment of hemophilia; and concentrations of platelets, used to control thrombocytopenic bleeding.  
      Along with the growing demand for these blood components, there is also a growing expectation for purity of the blood product. Before storing blood components such as red blood cells or platelets for later transfusion, it is believed to be desirable to minimize the presence of impurities or other materials that may cause undesired side effects in the recipient.  
      For example, it is generally considered desirable to remove leukocytes from such blood components before storage, or at least before transfusion. It is also believed beneficial that potential blood-born pathogens, e.g., viruses and bacteria, be inactivated from blood components prior to transfusion, e.g., through the use of photoactive and non-photoactive chemical reactions.  
     SUMMARY OF THE INVENTION  
      The invention provides systems and methods for processing concentrated red blood cells and the like. The systems and methods condition the concentrated red blood cells for subsequent pathogen inactivation processes prior to long term storage and/or transfusion.  
      The systems and methods condition a collected concentration of red blood cells (called “packed red blood cells or pRBC&#39;s) for a pathogen inactivation function, which removes and/or inactivates suspected pathogens prior to long term storage. The systems and methods include a leukocyte reducing function, which reduces the residual population of leukocytes in the pRBC&#39;s prior to pathogen inactivation. In one embodiment, the leukocyte removing function is accomplished by filtration. The systems and methods also add a synthetic conditioning solution to the pRBC&#39;s. The conditioning solution is selected to specially condition the pRBC&#39;s for pathogen inactivation, in terms of, e.g., desired viscosity and/or desired physiologic conditions, such as pH. The systems and methods include a dilution function, during which at least one component of a conditioning solution is mixed with the pRBC&#39;s prior to the leukocyte reducing function. The component lowers the viscosity of the pRBC&#39;s and can lead to higher flow rates during filtration.  
      It has been discovered that, when the component of the conditioning solution, as conventionally formulated, is added to the pRBC&#39;s during the dilution function, a degradation of filtration efficiencies results when certain filtration media are used. When such filtration media are used, there is an observed increase in the residual leukocyte population and/or a decrease in the recovery following filtration of the pRBC&#39;s after filtration in the presence of this conventionally formulated conditioning solution component, when compared to the residual leukocyte population and/or pRBC recovery in red blood cells filtered by the same filters when mixed with other typical additive solutions commercially available today.  
      Based upon this surprising discovery, it is believed desirable to differentiate between filtration media having performance characteristics that are adversely affected by exposure to the conventionally formulated component of the conditioning solution (which will be called Category A Filtration Media) from filtration media that are not adversely affected (which will be called Category B Filtration Media).  
      Accordingly, one aspect of the invention provides systems and methods that include an identification function, by which filtration media having performance characteristics that are adversely affected by exposure to the conventionally formulated conditioning solution (Category A Filtration Media) are identified and differentiated from filtration media that are not so adversely affected (Category B Filtration Media). Once identified, Category B Filtration Media can be selected to perform the leukocyte reducing function, if desired.  
      However, Category A Filtration Media can still be selected to perform the leukocyte reducing function, if desired. According to this aspect of the invention, the systems and methods counteract the expected degradation of leukocyte removal efficiency in various ways.  
      For example, the systems and methods can provide a higher osmolarity for the component of the conditioning solution added before and/or during filtration. A higher osmolarity means exposure of the pRBC&#39;s to less hypotonic conditions prior to and/or during filtration. Reducing the hypotenicity of the conditioning solution component can be accomplished in various ways, e.g. by the addition of dextrose, and/or the addition of sodium chloride, and/or by the retention of greater volumes of anticoagulated plasma with the pRBC&#39;s.  
      As another example, the systems and methods can raise the extracellular pH of the pRBC&#39;s prior to and/or during the leukocyte reducing function. The pH can be elevated in various ways, e.g., by revising the content of phosphate in the conditioning solution, or chilling the pRBC&#39;s prior to filtration.  
      As another example, the systems and methods can meter the introduction of the hypotonic component of the conditioning solution during the leukocyte reducing function. In this arrangement, exposure of the pRBC&#39;s to the component is reduced prior to and/or during the leukocyte reducing function.  
      Another aspect of the invention provides systems and methods that include a pump to convey pRBC&#39;s through the filter during the leukocyte reducing function. The pump reduces the time of exposure during filtration to conditions that may possibly degrade filtration efficiencies. Improved leukocyte-reduction may also result when a pump is used, due to the effect of pump-induced shear forces on the blood, which can stimulate platelet and/or leukocyte adhesion to the filtration media. 
    
    
     DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagrammatic view of a system and related method for treating a collected concentration of red blood cells to remove and/or inactivate suspected pathogens prior to long term storage and/or transfusion to a patient.  
       FIG. 2  is a view of a representative system for carrying out a leukocyte reducing function and the conditioning function of the method shown in  FIG. 1  using a hypotonic Esol-A component.  
       FIG. 3  is a view of a representative system for carrying out a dilution function of the method shown in  FIG. 1 , which coordinates a conditioning function using a hypotonic Esol-A component with a leukocyte reducing function.  
       FIG. 4  is a view of a representative system for carrying out a dilution function of the method shown in  FIG. 1 , which coordinates a conditioning function using a modified, less hypotonic (e.g., having a higher ostmotic strength) Esol-A component with a leukocyte reducing function.  
       FIG. 5  is a view of a representative system for carrying out a dilution function of the method shown in  FIG. 1 , which coordinates a conditioning function with a leukocyte reducing function, and which includes the use of an additive solution that is ultimately replaced by a hypotonic Esol-A component.  
       FIG. 6  is a view of a representative system for carrying out a conditioning function of the method shown in  FIG. 1  on a pre-processed, leukocyte-reduced pRBC unit.  
       FIG. 7  is a view of a representative system for carrying out a dilution function of the method shown in  FIG. 1 , which coordinates a conditioning function using a hypotonic Esol-A component with a leukocyte reducing function, and which includes pump-assisted flow.  
       FIG. 8  is a view of a representative system for carrying out a dilution function of the method shown in  FIG. 1 , which coordinates a conditioning function using a hypotonic Esol-A component with a leukocyte reducing function, and which includes metered flow of the hypotonic Esol-A component. 
    
    
      The invention is not limited to the details of the construction and the arrangements of parts set forth in the following description or shown in the drawings. The invention can be practiced in other embodiments and in various other ways. The terminology and phrases are used for description and should not be regarded as limiting.  
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      I. System Overview  
       FIG. 1  shows an overview of a system and related method  10  for treating a collected concentration of red blood cells B which can also be called Apacked@ red blood cells, or pRBC—to remove and/or inactivate suspected pathogens prior to long term storage and/or transfusion to a patient. Blood-borne pathogens can include a multitude of bacterial and/or viral agents such as, for example, hepatitus B virus or human immunodeficiency virus. It is not desirable to expose a patient in need of blood transfusion therapy to such pathogens, and it is the purpose of the system and related method  10  to minimize the possibility of this event.  
      The system and related method  10  include a blood separating function  12 . The function  12  processes whole blood drawn from a donor  14  to generate a pRBC unit  16 . The function  12  can comprise the use of conventional closed, sterile, manually manipulated blood collection systems, such as the Blood Pack® Units manufactured and sold by Baxter Healthcare Corporation, Deerfield, Ill. The use of manual systems will typically yield a pRBC unit  16  containing a unit volume of pRBC&#39;s of about 150 to 300 ml. Alternatively, the function  12  can comprise the use of automated blood collection systems, such as the Amicus™ Blood Collection System, or the Alyx™ Blood Collection System, of the CS-30007 Blood Collection System, all of which are manufactured and sold by Baxter Healthcare Corporation, Deerfield, Ill. The use the Alyx™ Blood Collection System can yield a unit volume of pRBC&#39;s of upwards to about 500 ml, although the exact yield can vary. Further details of the system and related method  10  in these different blood collection environments will be described later.  
      The system and related method  10  also include a leukocyte reducing function  18 . Reducing the residual population of leukocytes in a blood product collected for transfusion is recognized to be beneficial and is today recommended by most governmental agencies overseeing blood banking activities. For example, in the United States, for pRBC&#39;s to be considered “leukoreduced,” the regulatory requirement is the reduction of the residual population of leukocytes in a given pRBC unit to less than 5×106 prior to transfusion. The aggressive reduction of leukocytes also serves the beneficial purpose of removing the total pathogen load of the blood component by first removing pathogens that may be entrained within leukocytes prior to pathogen inactivation.  
      In the illustrated embodiment (as will be explained later), the pRBC unit  16  is desirably passed through a filter to separate leukocytes from the red blood cells, e.g., by exclusion using a membrane or by depth filtration through a fibrous filter media. It should be appreciated, however, that leukocyte separation can occur by various centrifugal and non-centrifugal techniques, and not merely “filtration” in the technical sense. Separation can occur by absorption, columns, chemical, electrical, and electromagnetic means. “Filtration” is broadly used in this specification and encompasses all of these separation techniques as well.  
      The system and related method also include a conditioning function  20 . It is desirable that the pRBC unit  16  be in a condition that facilitates a subsequent pathogen inactivation process, as well as long-term storage following pathogen inactivation. To this end, the conditioning function  20  adds a selected conditioning solution  22  to the pRBC unit  16 . The conditioning solution  22  is selected to specially condition the pRBC unit  16  for pathogen inactivation in terms of, e.g., desired viscosity and/or desired physiologic conditions, such as pH, which are conducive to effective pathogen inactivation. The conditioning solution  22  also desirably conditions the pRBC unit  16  for long-term storage after pathogen inactivation, by providing the proper mix of nutrients and buffers to sustain blood cell metabolism during storage.  
      By way of example, a conditioning solution  22  of the type known as Erythro-sol™ (also known as E-Sol™) (sold by Baxter Healthcare Corporation), can be mixed with the pRBC unit  16  to condition it for pathogen inactivation, particularly when the selected pathogen inactivation agent includes the frangible compounds disclosed in Cook et al. U.S. Pat. No. 6,093,725. As conventionally formulated, E-Sol™ Solution comprises sodium citrate (25 mM); dibasic sodium phosphate (16.0 mM); monobasic sodium phosphate (4.4 mM); adenine (1.5 mM); mannitol (39.9 mM); and dextrose (45.4 mM). For practical reasons related to heat sterilization (more particularly, because dextrose will degrade under heat sterilization conditions if maintained at a pH above 7.0, which is the pH condition of the overall Esol™ Solution, which typically ranges from 7.0 to 7.5 and is preferably between 7.3 to 7.5), the conventional E-Sol™ Solution is typically added to red blood cells as two separate components—an Esol-A component and a dextrose solution (called an Esol-B component). As conventionally formulated, the Esol A component comprises 94 ml of sodium citrate (26.6 mM); dibasic sodium phosphate (17.0 mM); monobasic sodium phosphate (4.7 mM); adenine (1.6 mM); and mannitol (42.5 mM). The above compositions can be made by modifying the stated concentrations by +/−15%. The pH of the Esol A component generally matches the pH of the overall E-sol™ Solution. However, being free of dextrose, the Esol-A component can undergo heat sterilization. The Esol-B component comprises 20 ml of 8% dextrose. Being acidic, the Esol-B component can undergo heat sterilization separate from the components of the Esol-A component.  
      The system and related method  10  also include a pathogen inactivating compound mixing function  24 . The mixing function  24  receives a pRBC unit  16  after it has undergone the leukocyte reducing function  18 . When a two-part conditioning solution is used, the mixing function  24  can also be preceded by at least a portion of the conditioning function  20 , which can occur before or as part of the leukocyte reducing function  18 , as will be described in greater detail later. The mixing function  24  mixes the pRBC unit  16  with desired volume of a selected pathogen inactivating compound  26 . As will be described later, when a two-part conditioning solution is used, at least a part of the conditioning solution  22  can be used as a suspension agent for the pathogen inactivating compound, as will be described later. The mixing function  24  ultimately generates a pRBC treatment unit, which comprises the leukocyte reduced pRBC unit  16  mixed with the conditioning solution  22  and pathogen inactivating compound  26 .  
      In the illustrated embodiment, in which the conditioning solution  22  comprises the Esol™ solution, the pathogen inactivation compound  26  desirably comprises a pH-sensitive frangible anchor-linker-effecter (Frale) compound. This compound performs a pathogen inactivating function by irreversibly preventing the replication of DNA of blood borne pathogens. A pathogen inactivation compound  26  of this type is β-alanine, —N-(acridin-9-yl), 2-[bis(2-chloroethyl)amino]ethyl ester. This compound  26  and its use are described in more detail in U.S. Pat. Nos. 6,093,725 and 6,410,219 which are incorporated by reference herein.  
      A quenching agent, e.g., L-Glutathione, is desirably used in association with the pathogen inactivating compound  26  just described. L-Glutathione is a naturally occurring tripeptide that does not penetrate the red cell membrane and pathogen membrane and/or coat. The purpose of a quenching agent is to inhibit non-specific reaction of the pathogen inactivation compound  26  with nucleophiles other than DNA/RNA, as the mixing function  24  may provide excess pathogen inactivation compound  26  to assure complete reaction treatment of the red cells.  
      The system and related method  10  also includes a pathogen inactivating function  30 . The pathogen inactivating function  30  carries out the steps required to complete the pathogen inactivation process of the pRBC treatment unit.  
      As above described, the pathogen inactivation compound  26  is pH-activated. At an acidic pH, the Frale compound  26  is inactive, and does not react excessively with the quenching agent. It can therefore be stored prior to use in an inactive state. However, when the compound  26  is added to the higher pH red cells during the mixing function  24 , the pathogen inactivating compound  26  becomes activated to carry out the inactivation process, which carries forward into the inactivating function  30 . Once activated by the proper pH conditions, the pathogen inactivation compound  26  becomes a highly reactive acridine based compound. During the subsequent inactivating function  30 , the activated compound penetrates the red cell membrane, pathogen membrane and/or coat and, through a reactive intermediate, cross links the nucleic acids and pathogens. The cross links inactivate the pathogens by preventing replication of their genomes.  
      The pathogen inactivating function  30  desirably subjects the combined collected red blood cells, inactivation compound  26 , and quenching agent (along with the conditioning solution  22 ) to further mixing, e.g., by passage through a static mixer. This assures that all the red blood cells have been treated by the inactivation agent. After mixing, the pathogen inactivating function  30  desirably also incubates the combined collected red cells, pathogen inactivation compound  26 , and quenching agent (along with the conditioning solution  22 ) for a period of time sufficient to assure that the inactivation process has taken place. Presently, it is contemplated that incubation in an environmentally controlled area having an ambient temperature of between about 19-25° C., for about 1-24 hours and more preferably 12 hours, is sufficient, although more or less time may be required. After incubation, the pathogen inactivating function  30  desirably treats the combined red cell and inactivation compound  26  to remove any unused pathogen inactivation compound  26  and any reaction or degradation product of the compound  26 . In a preferred embodiment, the combined solution is contacted with a sorption device, which may operate by adsorption, absorption or other sorption mechanisms to cleave to or otherwise remove any remaining inactivation agent and degradation or reaction by-products.  
      The pathogen inactivating function  30  ultimately generates a pathogen inactivated PRBC unit  32 . The pathogen inactivated unit  32  is ready to undergo long term storage and/or transfusion to a patient.  
      II. Coordination of the Conditioning Function  
      Desirably synergistic effects can be achieved by the purposeful coordination of the conditioning function  20  with the mixing function  24  and/or the leukocyte reducing function  18 .  
      A. Coordination of the Conditioning Function and the Mixing Function  
      As described above, the pathogen inactivation compound  26  may be provided in an inactive, ready-to-use liquid form or may alternatively be provided in a concentrated form which requires reconstitution or other processing before addition to the red cells. For example, the pathogen inactivation compound  26  as above described may be provided in the form of a crystalline powder, a granulated powder, tablet, capsule, lyophilized powder, concentrated liquid or frozen liquid. The compound  26  may be supplied in a wide variety of containers, such as bags, vials, rigid or flexible, syringe, or tubing or other appropriate container. In a preferred embodiment, about 10-100 mg, and more preferably about 50 mg of the compound  26  in dry powder form is contained in a vial or other suitable container. The quenching agent L-Glutathione may also be provided in various formulations and forms, including crystalline powder, liquid, low pH liquid, granulated powder, tablet, capsule, lyophilized powder or frozen liquid and may come in the same variety of containers as the pathogen inactivation agent. In a preferred embodiment as now contemplated, about 250-400 mg, and more preferably about 312 mg of L-Glutathione are provided in a vial or other suitable container.  
      The system and related method  10  shown in  FIG. 1  include a suspension function  28 . The suspension function  28  coordinates the conditioning function  20  with the mixing function  24 .  
      The suspension function  28  introduces the acidic (low pH) Esol-B component into the pathogen inactivating compound  26  prior to the addition of the inactivating compound  26  (now suspended in the Esol-B component) to the pRBC unit  16  (now in a leukocyte-reduced state). The low-pH Esol-B component does not activate the pathogen inactivating compound  26 . The suspension function  28  thereby allows the conditioning function  20  to augment the mixing function  30 , by suspending the pathogen inactivation compound  26  (and the quenching agent) without activating the compound  26 .  
      The suspension function  28  desirably carries out reconstitution of the pathogen inactivating compound  26  by repeated circulating the low pH Esol-B component into and out of the vial or container carrying the dry inactivation compound  26 , until the compound  26  is suspended or dissolved in the Esol-B component. The inactivating compound  26  and Esol-B component are then repeatedly injected into and withdrawn from the vial of container holding the quenching agent, until the quenching agent is also resuspended in the Esol-B component. The Esol-B component with the inactivating compound  26  and quenching agent are now ready to be added to the pRBC&#39;s, which desirably have already been treated to remove leukocytes during the leukocyte reducing function  18 . Alternatively, the quenching agent can be reconstituted first, followed by the reconstitution of the pathogen inactivating compound  26 .  
      B. Coordination of the Conditioning Function and the Leukocyte Reducing Function  
      In a representative embodiment (see  FIG. 2 ), the leukocyte reducing function  18  can be accomplished by passing the pRBC unit  16  from a collection container  34  (in the presence of an appropriate anticoagulant) to an integrally connected transfer container  36  through an in line leukocyte filter  38 , e.g., by gravity flow. In this arrangement, the transfer container  34  holds the requisite volume of the Esol-A component  40 , which mixes with the leukocyte-reduced pRBC&#39;s conveyed into the transfer container  36 .  
      Alternatively (as shown in phantom lines in  FIG. 2 ), the Esol-A component  40  can be transferred into the transfer container  36  from an auxiliary container  42  after conveyance of pRBC&#39;s into the container  36  through the filter  38  (in the presence of an appropriate anticoagulant). In this arrangement, the auxiliary container  42  is preferably coupled (by an integral connection or by use of a sterile docking technique) upstream of the filter  38  (as shown in phantom lines in  FIG. 2 ), so that the Esol-A component  40  passes through the filter  38 , flushing residual pRBC&#39;s from the filter  38 , and mixing with the pRBC&#39;s in the transfer container  36 .  
      In either situation, the Esol-B component is separately used as a suspension agent for the pathogen inactivating compound  26  and quenching agent, which is activated by its addition to the pRBC&#39;s and high pH Esol-A component  40  residing in the container  36  during the subsequent mixing function  24 , as has been previously described.  
      It is desirable to lower the viscosity of the packed red blood cells before passage through the filter  38 . The lower viscosity leads to higher flow rates, higher recovery of red blood cells, and a lower incidence of red blood cell damage or hemolysis. It is therefore desirable to add a viscosity-reducing solution to the red blood cells prior to filtration.  
      The system and related method  10  shown in  FIG. 1  includes a dilution function  44 . The dilution function  44  coordinates the conditioning function  20  with the leukocyte reducing function  18 , by introducing all or at least a portion of the Esol-A component to the pRBC unit  16  prior to and/or during the leukocyte reducing function  18 . The dilution function  44  thereby allows the conditioning function  20  to augment the leukocyte reducing function  18 , by reducing the red blood cell viscosity during the leukocyte reducing function  18 , while also providing at least some of the conditioning effect of the conditioning function  20 .  
      In a representative embodiment (see  FIG. 3 ), to carry out the dilution function  44 , an auxiliary container  42  holding the Esol-A component  40  can be coupled (either by an integral connection or by use of a sterile docking technique) to the collection container  34 . As before described, the collection container  34  is itself connected to the transfer container  36  through the in-line leukocyte filter  38  (either by an integral connection or by use of a sterile docking technique). In this arrangement, the Esol-A component  40  is added to the pRBC unit  16  in the collection container  34  prior to filtration, thereby carrying the dilution function  44  in this embodiment. It is the lowered viscosity mixture of Esol-A component  40  and red blood cells that is passed through the filter  38  into the transfer container  36 , e.g., by gravity flow.  
      In this arrangement, the Esol-B component is still used as the suspension agent for the pathogen inactivating compound  26  and quenching agent. The Esol-B component is added to the pRBC&#39;s and high pH Esol-A component  40  residing in the container  36  during the subsequent mixing function  24 , as has been previously described.  
      Surprisingly, it has been discovered that, for certain types of leukocyte filtration media, the addition of the Esol-A component  40 , as conventionally composed, to pRBC&#39;s prior to filtration, can result in a degradation of filtration efficiencies, such that there is an observed increase in the residual leukocyte population of the Esol-A pRBC&#39;s mixture after filtration, when compared to the residual leukocyte population in PRBC&#39;s filtered by the same filters when mixed with other typical additive solutions commercially available today.  
      For example, it has been observed that the residual leukocyte population in packed red blood cells is greater than expected and/or the RBC recovery is lower than expected when the pRBC&#39;s are filtered mixed with the Esol-A component through the fibrous depth filtration media contained in red blood cell filters manufactured by Asahi Medical Corporation—bearing the trade identifications RS-2000; RZ-400; Flex RC, etc.—compared to the residual leukocyte population and/or RBC recovery in red blood cells filtered by the same media mixed with conventional Adsol additive solution (manufactured by Baxter Healthcare Corporation). While the use of Adsol® additive solution prior to filtration results in acceptable reduced residual leukocyte populations and/or RBC recovery in pRBC&#39;s after filtration, Adsol® additive solution does not provide all the desired conditioning effects that Esol™ Solution provides. The use of Esol™ Solution is therefore still desired. Conversely, it has been observed that the residual leukocyte population and/or RBC recovery in packed red blood cells is not significantly different when the pRBC&#39;s are filtered mixed with the Esol-A component through the fibrous depth filtration media contained in red blood cell filters manufactured by Pall Corporation—bearing the trade identifications RCM-1, RC2D, BPF-4, etc.—compared to the residual leukocyte population in red blood cells filtered by the same media mixed with conventional Adsol® additive solution. It has also been observed that the residual leukocyte population and/or RBC recovery in packed red blood cells is not significantly different when the pRBC&#39;s are filtered mixed with the Esol-A component through the non-fibrous membrane filtration media contained in red blood cell filters manufactured by Terumo Corporation—bearing the trade identifications Imugard-III-RC—compared to the residual leukocyte population and/or RBC recovery in red blood cells filtered by the same media mixed with conventional Adsol® additive solution. To achieve the full benefits of dilution function  44 , it is therefore desirable to first differentiate between filtration media having performance characteristics that are adversely affected by exposure to the Esol-A component  40  (which will be called Category A Filtration Media) from filtration media that is not adversely affected (which will be called Category B Filtration Media). This differentiation can be achieved by in vitro testing, as the following Example 1 demonstrates.  
     EXAMPLE 1  
     Identifying Category A and Category Filtration B Media  
      Units of pRBC&#39;s (270 ml) were obtained by conventional manual centrifugation techniques from whole blood. The pRBC&#39;s were mixed with conventional Esol™ Solution at a 2:1 volumetric ratio. The pRBC-Esol mixtures were passed through various filtration media at room temperature and with no hold time after Esol™ Solution addition. The following Table summarizes the data (PASS indicates that the residual leukocyte population and/or RBC recovery in red blood cells mixed with the Esol™ Solution met selected standards of numbers of residual leukocytes and/or percentage of RBC recovery that can be achieved when red blood cells are filtered mixed with Adsol® Solution—in this Example, the standards selected were (1) having a residual leukocyte level that was less than 1×106 per unit with 95% confidence, 95% of the time and (2) a RBC recovery that was not less than about 89%, assuming a minimum 270 ml RBC unit. FAIL indicates that the selected residual leukocyte population and/or RBC recovery standards were not met in red blood cells mixed with the Esol™ Solution):  
               TABLE 1                          Filtration of pRBC&#39;s Mixed with Esol ™ Solution                                             RBC   Residual                       Recovery   Leukocyte   Filtration               (%)   Population   Time           No.   Mean   (Mean)   (Mean       Filter   Tested   Range   Range   Min)   Disposition                                             Asahi   7   89   5.6 × 10 4     11   FAIL       RS-2000       88 to 90   7.18E4 to       Category A                   1.26E6       Asahi   8   89   2.0 × 10 6     18   FAIL       Flex-RC       88 to 91   1.31E4 to       Category A                   1.10E7       Miramed   7   90   5.05 × 10 8     11   FAIL       Leuco-       89 to 91   1.29E8 to       Category A       stop 4           1.19E9       LDSS Mono       Miramed   5   90   7.71 × 10 8     9   FAIL       HE1 LDSS       89 to 91   3.2E8 to       Category A       Mono           1.64E9       Hemasure   5   93   1.73 × 10 6     12   FAIL       r/LS       92 to 93   8.28E4 to       Category A                   5.85E6       Fresenius   5   90   1.47 × 10 7     9   FAIL       BioR-       90 to 91   4.46E5 to       Category A       01MaxBS           4.68E7       Pall   5   94   1.64 × 10 5     17   PASS       RCM1       93 to 95   1.66E4 to       Category B                   4.89E5       Pall   10   88   6.94 × 10 4     19   PASS       RC2D       87 to 90   1.44E4 to       Category B                   1.5 E5       Terumo   22   89   4.84 × 10 4     7   PASS       Imugard       86 to 98   8.79E2 to       Category B       III-RC           5.32E5                  
 
      It is believed that Category B Filtration Media may have inherently different mechanisms for leukocyte adhesion than Category A Filtration Media, which results in different filtration efficiencies under certain special conditions.  
     1. Using Category A Filtration Media  
      When a Category A Filtration Media is selected for use, the dilution function  44  desirably includes systems and related methods that mediate the exposure of the pRBC unit  16  to the Esol-A component  40  (or like component) prior to and/or during filtration. This mediation can be accomplished in various ways.  
      a. Lowering the Hypotonicity of Esol-A  
      A potential cause of the phenomenon discussed above for Catagory A Filtration Media is believed to be related to the hypotenicity of the conventional composition of the Esol-A component  40 . Due to the absence of dextrose, the hypotonicity of the Esol-A component  40  is significantly greater than the hypotoncity of solutions containing dextrose, for example Adsol® Solution. Expressed in terms of osmolarity, the dextrose-free Esol-A component has an osmolarity of 178 mOsm/kg, whereas Adsol® Solution (containing 20.0 g/L of dextrose) has an osmolarity of 466 mOsm/kg). It is believed that the hypotenicity of the conventional composition of the Esol-A component due to the absence of dextrose may evoke physiologic changes in the morphology of the red blood cells and/or leukocytes, which ultimately affect the selective adsorption and/or flow dynamics of the Category A Filtration Media.  
      To make possible the synergistic coordination of the leukocyte reducing and conditioning functions  18  and  20 , the composition of the Esol-A component  40  can be modified to present a lower hypotenicity. The lowered hypotenicity can be achieved in various ways. One such way is by adding a selected amount of D-glucose anhydrous or monohydrate (dextrose) to the other Esol-A ingredients.  
     EXAMPLE 2  
     Addition of Dextrose Leads to Improved Leukocyte Reduction During Filtration  
      pRBC&#39;s from a pooled source were mixed with various additive solutions at a 2:1 volumetric ratio. Filtration through an Asahi Flex RC™ filter (a Category A Filtration Medium) began five to six minutes after addition of solution. The following table shows the results:  
               TABLE 2                          Filtration of pRBC&#39;s       Mixed with Esol-A Solution       Modified with Dextrose                                         Modified Esol-A               Esol-A Solution   Solution           Composition   (No Dextrose)   (With Dextrose)                                             Mannitol   7.74   7.74           g/L           Adenine   0.215   0.215           g/L           Dextrose   0.0   20.0           (anhydrous)           g/L           Sodium   7.82   7.82           Citrate           (dihydrate)           g/L           Sodium   .649   .649           Phosphate           (monohydrate)           g/L           Sodium   2.42   2.42           Phospate           Dibasic           (anhydrous)           Osmolarity   178   293           mOsm/kg           Initial   1.86 × 10 9     1.86 × 10 9             Leukocyte           Population           per 270 ml           Post-   2.43 × 10 5     4.05 × 10 4             Filtration           Leukocyte           Population           per 270 ml                      
 
      Table 2 demonstrates that the addition of dextrose to Esol-A Solution leads to improved leukocyte removal rates using a Category A Filtration Medium.  
      A representative composition for a hypotonicity-modified Esol-A component is: 94 ml of sodium citrate (26.6 mM); dibasic sodium phosphate (17.0 mM); monobasic sodium phosphate (4.7 mM); adenine (1.6 mM); mannitol (42.5 mM); and D-glucose monohydrate (20 to about 86 mM). Due to the heat sterilization considerations previously described, the modified Esol-A composition is desirably provided in two parts. In a representative embodiment shown in  FIG. 4 , two containers  46  and  48  can be coupled (either by an integral connection or by use of a sterile docking technique) to the collection container  34 .  
      The container  46  holds a low pH D-glucose component  50 —comprising, e.g., about 24 to 70 mL of (about 94/24 to 94/70) of D-glucose monohydrate (20 to about 86 mM). This component  50  can withstand heat sterilization.  
      The container  48  holds the remaining high pH components  52 —comprising, e.g., about 50 to 70 mL of (about 94/50 to 94/70) of sodium citrate (26.6 mM); dibasic sodium phosphate (17.0 mM); monobasic sodium phosphate (4.7 mM); adenine (1.6 mM); and mannitol (42.5 mM).  
      As before described (and as shown in  FIG. 4 ), the collection container  34  is itself connected to the transfer container  36  through the in-line leukocyte filter  38  (either by an integral connection or by use of a sterile docking technique). In this arrangement, the Esol-A components  50  and  52  are separately added to the pRBC unit  16  in the collection container  34  and mixed prior to filtration. It is the lowered viscosity mixture of Esol-A components  50  and  52  and red blood cells that is passed through the filter  38  into the transfer container  36 .  
      In this arrangement, the composition of the Esol-B component is modified to result, after mixing with the modified Esol-A components  50  and  52 , in the same overall final composition of the Esol conditioning solution  22 . In the described embodiment, the modified Esol-B component  54  comprises 20 ml of D-glucose (0 to about 305 mM) in water. The 20 mL of the modified Esol-B component  54  can be used as the suspension agent for the pathogen inactivating compound  26  and quenching agent, as before described. This suspension is added to the pRBC&#39;s and high pH Esol-A components  50  and  52  residing in the container  36  during the subsequent mixing function  24 , as has been previously described.  
      In this arrangement, all or part of the D-glucose desired in the ultimate conditioning solution  22  can be formulated in the modified low pH D-glucose component  50  in the container  46 , as required to keep the hypotenicity of the mixed hypotonicity-modified Esol-A component at a desired state. It is believed that this lessened degree of hypotenicity for the Esol-A component (and/or the preservation effect of dextrose itself) will facilitate the selective removal of leukocytes by Category A Filtration Media. In the event that all of the D-glucose is formulated in the component  52  in the container  46 , the water ingredient of the modified Esol-B component  54  (which would now be free of any D-glucose) could be partially or entirely replaced by a 0.9% saline or other salt solution. The substitution—entire or partial—of the saline or salt solution for water will “soften” the effects of adding the now more hypotonic Esol-B component resuspension of the pathogen inactivating compound  26  and quenching agent to the red blood cells.  
      Dextrose is known to rapidly cross cell membranes and therefore may not, on its face, be considered as contributing to “effective” osmolarity. Still, if the dextrose cannot equilibrate between the cells and the supernatent—and, as a result, the extracellular concentration remains greater than the intracellular concentration—then dextrose may provide some contribution to the “effective” osmolarity, even though it does not affect ionic strength.  
      To make possible the synergistic coordination of the leukocyte reducing function  18  and condition function  20 , the composition of the S-Sol-A component  40  can be modified to increase its ionic strength, as well contribute to osmolarity, by the addition of sodium chloride to Esol-A.  
     EXAMPLE 3  
     Addition of Sodium Chloride Leads to Improved Leukocyte Reduction During Filtration  
      pRBC&#39;s from a pooled source were mixed with various additive solutions at a 2:1 volumetric ratio. Filtration through an Asahi Flex RC™ filter (a Category A Filtration Medium) began five to six minutes after addition of solution. The following table shows the results:  
               TABLE 3                          Filtration of pRBC&#39;s       Mixed with Esol-A Solution                                         Modified Esol-           Modified with   Esol-A Solution   A Solution           Sodium Chloride   (No Sodium   (With Sodium           Composition   Chloride)   Chloride)                                             Mannitol   7.74   7.74           g/L           Adenine   0.215   0.215           g/L           Sodium Chloride   0.0   4.39           g/L           Sodium Citrate   7.82   7.82           (dihydrate) g/L           Sodium Phosphate   .649   .649           (monohydrate) g/L           Sodium Phospate   2.42   2.42           Dibasic           (anhydrous)           Osmolarity   178   315           mOsm/kg           Initial Leukocyte   1.86 × 10 9     1.97 × 10 9             Population           per 270 ml           Post-Filtration   2.43 × 10 5     5.40 × 10 4             Leukocyte           Population           per 270 ml                      
 
      Table 3 demonstrates that the addition of sodium chloride to Esol-A Solution leads to improved leukocyte removal rates when using a Category A Filtration Medium. Centrifugation conditions also may have an impact upon the magnitude of leukocyte reduction. If pRBC&#39;s are separated from whole blood at greater centrifugal forces (a so-called “hard spin”), a majority of platelets sediment into the pRBC&#39;s. This is contrasted with the separation of pRBC&#39;s at lower centrifugation forces (a so-called “soft spin”), during which a majority of the platelets remain in the supernatant. It has been discovered that use of a “hard spin” to harvest pRBC&#39;s optimizes the synergistic coordination of the leukocyte reducing function  18  and condition function  20 , particularly in conjunction with use of a dextrose-modified Esol-A Solution.  
     EXAMPLE 4  
     Centrifugation Conditions Affect Leukocyte Removal During Subsequent Filtration  
      Freshly collected whole blood was subject to centrifugation under both “hard spin” and “soft spin” conditions to yield pRBC&#39;s. The pRBC&#39;s were mixed with various additive solutions at a 2:1 volumetric ratio. Filtration through an Asahi Flex RC™ filter (a Category A Filtration Medium) began after addition of solution. The following table shows the results:  
               TABLE 4                          Filtration of pRBC&#39;s       Mixed with Various Additive Solutions       After Hard Spin and Soft Spin Conditions                                                 Mean Residual           Dextrose   Number       Post       Additive   Concentration   of       Filtration       Solution   in Additive   Units   Centrifugation   Leukocyte       Composition   Solution   Tested   Conditions   Population               Esol-A   0   5   Hard Spin   3.26 × 10 5         Esol-A   0   6   Soft Spin   1.52 × 10 6         Esol-A with   35 mM   4   Hard Spin   1.40 × 10 5         Dextrose       Esol-A with   35 mM   6   Soft Spin   4.10 × 10 4         Dextrose       Esol-A with   71 mM   5   Hard Spin   5.12 × 10 4         Dextrose       Esol-A with   71 mM   5   Soft Spin   6.39 × 10 4         Dextrose       CPD Plasma   Conventional   5   Hard Spin   2.16 × 10 5             Formulation                  
 
      Table 4 demonstrates that leukodepletion of pRBC&#39;s can be enhanced by the presence of platelets, which is the result of a hard spin. Table 4 also underscores that the addition of dextrose improves leukocyte reduction performance of a Category A Filtration Medium.  
      As Table 4 also demonstrates, the pRBC unit may also be modified by retaining a greater volume of anticoagulated CPD plasma, resulting in improved removal of leukocytes using a Category A Filtration Medium. The presence of a greater volume of CPD or ACD plasma elevates the osmolarity (reducing hypotonicity) of the blood product, and also provides dextrose for metabolic function.  
      b. Use of a Less Hypotonic Solution During Filtration, Replaced by Esol-A After Filtration  
      In another embodiment (see  FIG. 5 ), a synergistic coordination of the leukocyte reducing and conditioning functions  18  and  20  is made possible by using a less hypotonic additive solution  56  to dilute the pRBC&#39;s during filtration using Category A Filtration Media. This additive solution  56  is selected, not necessarily to provide any conditioning for a subsequent pathogen inactivating function, but rather to provide an environment where optimal removal of leukocytes occurs during the leukocyte removal function  18  using the Category A Filtration Media. For example, the additive solution  56  can comprise conventional Adsol® Solution or SAG-M™ Solution.  
      In this arrangement, the conditioning function  20  includes the removal of the additive solution  56  following filtration and the subsequent addition of the Esol-A component  40 , as currently formulated, to the filtered pRBC unit  16 .  
      To carry out this process (as a representative embodiment of  FIG. 5  shows), the collection container  34  for the pRBC unit  16  can be coupled to the transfer container  36  via the in-line filter  38 , as previously described. In this representative embodiment, the transfer container  36  holds the non-conditioning additive solution  56 . The additive solution  56  is transferred, e.g., by gravity flow through a one-way by-pass path  58 , into the collection container  34 . The additive solution  56  is mixed with the pRBC unit  16  in the collection container  34 . After mixing, the pRBC unit  16  and additive solution  56  are conveyed through the filter  38  into the transfer container  36 , e.g., by gravity flow. The transfer container  36  is separated from the filter  38 , and the filtered pRBC and additive solution  56  are centrifugally separated within the transfer container  36 . The additive solution  56  is expressed from the container  36  (e.g., into another container or—if the transfer container  36  is not disconnected from the container  34  before centrifugation—into the container  34 , leading the filtered pRBC unit behind in the container  36 . The Esol-A component  40 , as presently formulated, can now be transferred into the transfer container  36  from an auxiliary container  60  where it is mixed with the pRBC unit  16 . The auxiliary container  60  can be either integrally connected by tubing to the transfer container  36  or connected by a sterile docketing technique.  
      In this arrangement, the Esol-B component, as presently formulated, can be used as the suspension agent for the pathogen inactivating compound  26  and quenching agent, and added to the pRBC&#39;s and high pH Esol-A component  40  residing in the container  36  during the subsequent mixing function  24 , as has been previously described.  
      In an alternative embodiment shown in  FIG. 6 , a collection container  74  may include a pre-processed pRBC unit  76  that has already undergone leukocyte reduction and to which an additive solution  78  (like Adsol® Solution) has already been added. The pre-processed unit  76  may be provided, e.g., as a result of processing by the automated Alyx™ Blood Separation System, or by the automated Trima™ Blood Separation System sold by Cobe Laboratories (a division of Gambro), or by the MSC Plus™ Blood Separation System sold by Haemonetics Corporation. It may be desirable to subject the pre-processed pRBC unit  76  to the conditioning function  24 , to convert the pre-processed, leukocyte-reduced unit  76  into a pRBC unit  16  conditioned for a subsequent pathogen inactivation function  30 .  
      In this embodiment, a conditioning conversion assembly  80  can be provided comprising an integrally connected empty container  82  and a container  84  holding the Esol-A component  40 . The conditioning conversion assembly  80  is integrally attached or coupled to the collection container  74  by a sterile docking technique, after the collection container  74  has undergone centrifugation to separate the additive solution  78  from the pre-processed pRBC&#39;s. The centrifugally separated additive solution  78  is conveyed into the empty container  82 , and the Esol-A component  40  is transferred into the collection container  74  to resuspend the pRBC&#39;s.  
      In an alternative arrangement, it may be desirable to add additional solution to the pre-processed pRBC unit  76  to decrease the viscosity of the pRBC&#39;s before subjecting the unit  76  to centrifugal separation. In this arrangement, the empty container  82  could hold a supplemental volume of an additive solution  86  (shown in phantom lines in  FIG. 6 ) B e.g. Adsol® Solution or the modified low pH D-glucose component  50 , described above. This solution  86  is added to the pRBC unit  76 , and the unit  76  is then subject to centrifugal separation. Following centrifugal separation, the additive solution  78  and supplemental solution  86  are conveyed into the container  82  (which is now empty), and the Esol-A component  40  is conveyed from the container  84  into the collection container  74 .  
      The Esol-B component, as presently formulated, can subsequently be used as the suspension agent for the pathogen inactivating compound  26  and quenching agent, and added to the pRBC&#39;s and high pH Esol-A component  40  residing in the container  74  during the subsequent mixing function  24 , as has been previously described.  
      c. Elevating the Extracellular pH  
      A potential cause of the phenomenon discussed above for Catagory A Filtration Media is believed to be related to the extracellular pH of pRBC&#39;s in the presence of the conventional composition of the Esol-A component  40 . pRBC&#39;s that are suspended in the Esol-A component  40  exhibit a lower extracellular pH (by approximately 0.1 to 0.2 pH units) than do pRBC&#39;s suspended in Adsol® Solution. It is believed that the lower pH may evoke physiologic changes in the morphology of the red blood cells and/or leukocytes, which ultimately affect the selective adsorption and/or flow dynamics of the Category A Filtration Media.  
      To make possible the synergistic coordination of the leukocyte reducing and conditioning functions  18  and  20 , the composition of the Esol-A component  40  can be modified to present a high pH. The pH may be elevated, e.g., by revising the content of phosphate frpm 4.7 mM sodium phosphate monobasic and 17.0 mM sodium phosphate dibasic to approximately 21.7 mM sodium phosphate dibasic. The ph also may be elevated, e.g., by chilling the pRBC&#39;s prior to filtration. Chilling red blood cells raises the pH of the blood product.  
      d. Use of a Pump to Reduce Time of Exposure to Hypotonic Conditions During Filtration  
      The leukocyte removing function  18  can rely upon gravity flow to convey the pRBC unit  16  through the filter  38 , as previously described. Even when the viscosity of the pRBC unit  16  is reduced by the addition of the Esol-A component  40  or other additive solution before filtration, the gravity flow filtration time can be as much as 30 minutes for pRBC&#39;s (240 mL to 360 mL) at ambient temperatures and as much as 360 minutes for pRBC&#39;s (240 mL to 360 mL) that have been refrigerated. The gravity flow filtration time also reflects the time that pRBC&#39;s are exposed to the Esol-A component  40 .  
      Regardless of the exact mechanism that is causing the observed degradation of filtration efficiencies for Category A Filtration Media, it is believed that a correlation exists between the time of exposure of the pRBC&#39;s to the component  40  prior to filtration and changes in filtration efficiencies that may result as a result of that exposure during filtration. It is therefore believed desirable to shorten the time of filtration, to thereby shorten the time of exposure. Shortening the filtration time is advantageous not only with respect to facilitating the use of Category A Filtration Media, but it also shortens the overall processing time, which is beneficial regardless of the type of filtration media that is selected for use.  
      In a representative embodiment shown in  FIG. 7 ), a synergistic coordination of the leukocyte reducing and conditioning functions  18  and  20  is made possible by using a pump  64  during the leukocyte reducing function  20 . The pump  64  conveys the pRBC unit  16  from the collection container  34  through the filter  38  and into the transfer container  36  at flow rates in excess of gravity flow rates. The pump  64  may comprise, e.g., a convention peristaltic pump or a diaphragm pump or a syringe-type pump. In this embodiment, an auxiliary container  62  can be coupled (either by an integral connection or by use of a sterile docking technique) to the collection container  34 . The container  62  holds the Esol-A component  40 , as presently formulated. All or part of the Esol-A component  40  is conveyed into the collection container  34  (e.g., by gravity flow) for mixing with the pRBC unit  16  just prior to the commencement of filtration. The pump  64  is actuated to pump the pRBC unit  16  and mixed component  40  through the filter  38  and into the transfer container  36  at a commanded flow rate. If only a portion of the Esol-A component  40  is placed into the collection container  34  prior to filtration, the remainder of the Esol-A component  40  may be conveyed through the filter  38  after filtration of the pRBC unit  16  is completed, to perform a filter-rinsing step in the process of being mixed with the filtered pRBC unit  16 .  
      Typically, commanded blood flow rates of upwards to 250 ml/min can be achieved using pumps without damage or hemolysis to red blood cells. Given a typical manually collected pRBC unit volume of 240 mL to 360 mL, the filtration time, and thus the overall time of exposure of the pRBC unit  16  to the conditions of the Esol-A component  40  during filtration, can be significantly reduced to less than two minutes.  
      Use of the pump  64  also promotes full exposure of the pRBC&#39;s to the entire surface area of the filtration media. More optimal leukocyte removal efficiencies can therefore be achieved, while also achieving the viscosity reducing benefits that the Esol-A component  40  provides, as well as also achieving the overall pathogen inactivation conditioning benefits that the combined Esol Solution provides. These benefits accrue from use of the pump  64  regardless of the type of filtration media that is selected for use.  
      Improved leukocyte-reduction may also result in the pump-driven arrangements just described due to the effect of pump-induced shear forces on the blood, which can stimulate platelet and/or leukocyte adhesion to the filtration media.  
      e. Metered Introduction of Hypotonic Esol-A Solution During Filtration  
      As previously described, about 94 ml of the Esol-A component  40  or other additive solution is added to the pRBC unit  16  before filtration to lower the viscosity of the pRBC unit  16  during filtration. The preceding embodiments mix the complete volume of the component  40  or additive solution to the pRBC unit  16  at one time before filtration. It is believed that the onset of reduced filtration efficiencies as a result of the exposure to the component  40  can also be mediated by metering the exposure of the pRBC&#39;s to the component  40  prior to and/or during filtration.  
      As shown in a representative embodiment in  FIG. 8 , the metering can be accomplished in a gravity flow system by conveying the Esol-A component  40  from a container  68  in a metered flow into the pRBC&#39;s prior to passage through the filter  38 . The metered flow can be controlled, e.g., by an in-line manual or proportional flow restrictor device  66  located between the container  68  and the junction  70  at which the Esol-A component  40  enters the flow of pRBC&#39;s through the filter  38 . The device  66  is desirably adjustable to select a desired flow rate ratio between pRBC&#39;s and Esol-A component  40  entering the filter  38 .  
      As shown in phantom lines in  FIG. 7 , the metering can also be accomplished in a pump-assisted system. In this arrangement (shown in phantom lines), the Esol-A component  40  is conveyed from a container  68  at a controlled flow rate through a pump  72  into the pRBC&#39;s prior to passage through the filter  38 . The flow rate of the pump  72  is desirably controlled relative to the flow rate of the pRBC pump  64  to provide the desired flow rate ratio between pRBC&#39;s and Esol-A component  40  entering the filter.  
      The metering of the Esol-A component  40  significantly reduces the time that pRBC&#39;s are exposed to the conditioning component  40  prior to filtration. Removal of leukocytes can therefore take place prior to the degradation of filtration efficiencies that may result due to that exposure.  
      Other advantages are obtained by the metering of the conditioning solution  22  into the pRBC&#39;s during the leukocyte reducing function  18 , regardless of the type of filtration media that is selected for use. By controlling the ratio of pRBC&#39;s and conditioning solution, the conditioning solution is always introduced at a constant desired ratio. Therefore, regardless of the volume of red blood cells collected, the final red blood cell/conditioning solution hematocrit can be constant. The metered supply of red blood cells and conditioning solution through the filter  38  eliminates the need to first drain the conditioning solution into the red blood cell collection container  34 , which lessens the overall procedure time. The metered supply of red blood cells and conditioning solution through the filter  38  also eliminates the need to manually agitate a red blood cell/conditioning solution mixture prior to leukofiltration. Due to density differences, when concentrated red blood cells are added to a conditioning solution, or vice versa, the conditioning solution floats to the top. Poorly mixed, high hematocrit, high viscosity red blood cells lead to reduced flow rates during leukofiltration. Poorly mixed, high hematocrit, high viscosity red blood cell conditions can also lead to hemolysis. By metering passage of red blood cells and conditioning solution through the filter  38 , mixing occurs automatically without operator involvement.  
     2. Using Category B Filtration Media  
      Category B Filtration Media can be identified using the characterization process previously described. The coordination of the conditioning function  20  and the leukocyte removing function  18  above described can thereby proceed using the Category B Filtration Media, without regard to mediating the exposure of the pRBC unit  16  to the Esol-A component  40  or like component prior to and/or during filtration, as above described. Category B Filtration Media, e.g., the Terumo RC IIIC, can be used in blood collection systems of the type shown in  FIGS. 2 and 3 , to carry out the leukocyte reducing function shown in  FIG. 1 , with or without the dilution function  44 .  
      Furthermore, the additional benefits of pump-assisted flow through the filter  68  and metering of conditioning solution into the pRBC&#39;s during leukocyte filtration remain desirable in their own right, even when using a Category B Filtration Media. Accordingly, Category B Filtration Media can be used in blood collection systems of the type shown in  FIGS. 7 and 8 .  
      It should now be apparent that the system and associated method  10  shown in  FIG. 1  lends itself to practice in either a manual way (e.g., by gravity flow and manual manipulation of the containers), or an automated way (e.g., using pump-assisted flow and in-line flow control devices coupled to the containers), or in a hybrid way incorporating both manual and automated control techniques.  
      The foregoing is considered as illustrative only of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. While the preferred embodiment has been described, the details may be changed without departing from the invention, which is defined by the claims.