Spin a test tube of blood in a centrifuge and it will separate into four layers: red cells, white cells, platelets, and plasma. Plasma is the most abundant, constituting a little over half of blood’s volume. It is more than 90 percent water with some salts, fats, and other chemicals suspended in it. That isn’t to say plasma is unimportant, however. It is anything but. Antibodies, clotting factors, and other constituent parts can be separated out and used in concentrated form to treat autoimmune diseases or hemophilia—and that is a huge business. In the United States, plasma sales make up 1.6
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