Any colour as long as it's red

THERE are 29 possible combinations of human blood groups, and for a patient to be given a safe transfusion the right one needs to be available. Besides the familiar ABO and rhesus systems, science has (after a sticky start, see article) uncovered the MNS system, the Kell system, the Lewis system and the D+ and D- group. Choose wrong—or settle for a less than perfect match because no other is to hand—and antibodies produced by the recipient's immune system may attack the foreign cells. That can lead to serious consequences. In the worst cases a patient's organs may fail and he may die.

Several attempts have been made to create artificial blood from chemicals that readily dissolve and transport oxygen, or to process the natural stuff in ways that eliminate the antigenic proteins which provoke this immune response. None, so far, has succeeded. But that has not stopped people trying. The latest effort, reported in Biomacromolecules by Maryam Tabrizian of McGill University in Montreal and her colleagues, follows the second approach, but with a twist. Instead of purging the antigenic proteins from red blood cells, Dr Tabrizian covers them up.

In essence, what she is doing is painting the cells. And, like all good decorators, she uses two coats. The first—the undercoat—is a material that sticks both to the fatty surface membrane of the red cells and to the mixture Dr Tabrizian chose as the outer coat. Crucially, this second mixture is of no interest whatsoever to the immune system. Nor is anything buried beneath it. Like blemishes on a bathroom wall, the antigenic proteins become invisible when painted over.

In practice, as is often the case when decorating a house, more than two coats may be needed to cover things up. But the layers can be alternated to any thickness desired, and inconvenient antigens thus hidden from view.

And it seems to work. When Dr Tabrizian exposed the newly coated blood cells to antibodies that would normally have been expected to stick to them and cause them to gather in useless clumps, nothing happened. Nor does the coating appear to harm the cells. That is mainly because red blood cells do very little other than absorbing and releasing oxygen and carbon dioxide as appropriate. They have no nuclei and no metabolism worth speaking of, so there is little to go wrong.

Both types of paint are, though, permeable by oxygen and carbon dioxide, and that permeability means the red blood cells can still do their job. Initial tests monitoring the flow of oxygen in and out of the cells show that they are able to transfer the gas just as efficiently as cells that have not been tampered with.

What Dr Tabrizian and her colleagues have not yet done is put their new creation back in an animal. That is the next test. But if the cells do work in vivo as well as in vitro, the team will have taken a big step on the road to the much-desired goal of blood transfusion that no longer has to worry about matching blood groups.