Edward L Snyder, Yale University School of Medicine and Blood Bank/Apheresis Service, Yale-New Haven Hospital, New Haven, Connecticut, USA

Joan Judge, Department of Pathology, Norwalk Hospital, Norwalk, Connecticut, USA

Plasma is the noncellular fraction of blood which transports red cells, white cells and platelets. It is an aqueous solution of salts, proteins, lipids, sugars and various other substances serving a myriad of physiological functions. Plasma is obtained for clinical use either by centrifugation of anticoagulated whole blood and collection of the supernatant or as source plasma from a plasmapheresis program (see below). If plasma is allowed to clot, the residual liquid is called serum. Essentially, serum is plasma minus the clotting factors V, VIII and fibrinogen which are consumed during clot formation.

Plasma was first used as a treatment for shock in the 1930s after the development of sodium citrate anticoagulant made possible the separation of cellular blood components from plasma by centrifugation. After the development of lyophilization techniques, plasma from many donor units could be pooled and stored for an indefinite period at room temperature as a freeze-dried preparation to be reconstituted later with sterile water. Because pooled plasma was found to be associated with a high risk of viral transmission, in particular hepatitis B, this pooled lyophilized product is no longer used in current medical practice when plasma is needed. Rather, single donor units of fresh frozen plasma have become the standard therapeutic component. Given the small amount of some blood proteins in plasma, such as coagulation factors, obtaining commercial quantities of coagulation factor concentrates still requires use of large pools of source plasma. These products, however, generally undergo one or more viral inactivation steps during processing.

In the early 1900s, serum was used medically as a source of immunoglobulins to treat infections including diphtheria, rabies, anthrax and tetanus. The first immune serums to be used were collected from naturally or artificially infected cattle or sheep; however, the frequency of anaphylactic reactions to these products, after the first injection, made them unsuitable for most human uses. Today, purified human immunoglobulins are available and scrum is used mainly as a diagnostic laboratory reagent and in tissue culture media.

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