1. A method of separating a plasma protein or a mixture thereof from blood plasma, one of plasma fractions or their cryosupernatant comprsising contacting said blood plasma, one of plasma fractions or their cryosupernatant with a cellulose-based filter aid to produce a slurry and passing or pumping said slurry through a filter vessel or filter mesh to obtain a filtrate and filter cake. 2. A method of separating a plasma protein or a mixture thereof from blood plasma, one of plasma fractions or their cryosupernatant comprising the steps of: (i) precoating a filter mesh with a cellulose-based filter aid; and (ii) passing or pumping said mixture from blood plasma, one of plasma fractions or their cryosupernatant  through the precoated filtermesh to obtain a filtrate and a filter cake. 3. The method according to claim 2, wherein additional cellulose-based filter aid is added to the mixture of said blood plasma, one of plasma fractions or their cryosupernatant through the precoated filter mesh. 4. The method according to any one of claims 1 to 3, comprising the further step of recirculating the filtrate one or more times, into the feed or filter vessel. 5. The method according to any one of claims 1 to 4, comprising the further step of washing or flushing the filter cake with a suitable solvent or aqueous buffer solution to remove residual mother liquor derived from the feed mixture. 6. The method according to claim 5, wherein the step of washing or flushing the filter cake involves up to three filter vessel volumes of the solvent or aqueous buffer solution. 7. The method according to any one of claims 1 to 6, wherein the concentration of cellulose-based filter aid is up to about 2.0% (weight per unit volume of feed mixture or weight per unit weight of feed mixture). 8. The method according to claim 7, wherein the concentration of cellulose-based filter aid is in the range of about 0.5% to about 2.0% (weight per unit volume of feed mixture or weight per unit weight of feed mixture). 9. The method according to any one of claims 1 to 8, wherein the plasma contains fresh-frozen plasma, non-fresh frozen plasma and the plasma fraction includes an intermediate Cohn fraction or Oncley fraction. 10. The method according to claim 9 wherein the plasma protein is selected from the list comprising albumin, immunoglobulin, lipoprotein, euglobulin, factorVIII, prothrombin complex, antithrombin III or other components of blood, amongst others, in a precipitated or complexed or aggregated form or bound to an insoluble carrier such as, but not limited to, fumed silica. 11. The method according to claim 10, wherein the cellulose-based filter aid possesses one or more of the characteristics: (i) it facilitates the flow of the feed mixture through the filter mesh during the filtration process; (iii) it does not generates PKA levels in plasma-derived products which are above British Pharmacoporia (BP), European Pharmacoporia (EP) or United States Pharmacoporia (USP) standards; and (iv) it does not leach aluminium into plasma-derived products at levels which are above BP, EP or USP standards. 12. The method according to claim 11, wherein the cellulose-based filter aid is selected from the list comprising Diacel<(TM)> 150, Diacel<(TM)> 200, Arbocel<(TM)> 200 or Vitacel<(TM)> 200, amongst others. 13. A method of separation of a solid-phase material from blood, fresh-frozen plasma, nonfresh frozen plasma, cryosupernatant or plasma fraction derived therefrom, such as an intermediate Cohn fraction or Oncley fraction, or other plasma fraction, said method comprising at least one filtration step to recover said solid-phase material, wherein said filtration step is employed using a cellulose-based filter aid. 14. The method according to claim 13, wherein the solid-phase material is a plasma protein selected from the list comprising albumin, immunoglobulin, lipoprotein, euglobulin, factorVIII, prothrombin complex, antithrombin III or other components of blood, amongst others, in a precipitated or complexed or aggregated form or bound to an insoluble carrier such as, but not limited to, fumed silica. 15. The method according to claim 13 or 14, preceded by at least one ethanol/acetate precipitation or treatment with fumed silica to produce the solid phase material. 16. An isolated plasma protein or isolated a mixture thereof produced according to the method defined by any one of claims 1 to 15. 17. The isolated plasma protein or isolated a mixture thereof according to claim 16, further characterized as a protein for therapeutic or prophylactic treatment of humans or mammals. 18. The isolated plasma protein or isolated a mixture thereof according to any one of claims 1 to 15 wherein said plasma protein is selected from the list comprising lipoprotein, euglobulin, immunoglobulin, factor VIII, prothrombin complex, antithrombin III, or albumin, and said mixture contains any or several of said plasma proteins. 19. The isolated plasma protein or isolated a mixture thereof according to any one of claims 1 to 15 wherein said plasma protein is selected from the list comprising albumin, lipoprotein, immunoglobulin or euglobulin and said mixture contains any or several of said plasma proteins. 20. The isolated albumin fraction produced according to any one of claims 1 to 15. 21. The isolated albumin fraction produced according to any one of claims 1 to 3.and 13 to 14 wherein the level of lipoprotein therein is below about 3.0% on a weight-for weight (albumin/lipoprotein) basis. 22. The isolated plasma protein or isolated a mixture thereof according to any one of claims 1 to 15 wherein the level of PKA and/or aluminium is a preparation of said plasma protein is low. 23. The isolated plasma protein or isolated a mixture thereof according to any one of claims 1 to 15 wherein the level of PKA and/or PKA-C1, esterase and/or kallikrein and/or aluminium is a preparation of said plasma protein or a mixture thereof is below the BP, EP or USP minimum standard. 24. The selected immunoglobulin produced according to any one of claims 1 to 15 wherein the level of aluminium in immunoglobulin preparations is below about 50 μg/L. 25. The selected immunoglobulin produced according to any one of claims 1 to 15 wherein the level of aluminium in immunoglobulin preparations is below about 10 μg/L. 26. The isolated albumin fraction produced according to any one of claims 1 to 15, wherein the level of PKA in albumin is below about 5 IU/mL. 27. The isolated albumin fraction produced according to any one of claims 1 to 15, wherein the level of PKA-C1 esterase in albumin preparations is below about 10 IU/mL.