RH negative blood is urgently needed in the city, according to Xie Yunzheng, manager of social services for the Shanghai (Red Cross) Blood Centre.

The centre has found only about 40 volunteers with Rh negative blood. "They are very warm-hearted to donate blood every year," he said. "But it is really a small pool considering the medical demand in the city."

With such a small amount, the hospitals in Shanghai frequently lack Rh negative blood. Some operations have to be delayed or even cancelled.

Rh blood types were discovered in 1940 by Karl Landsteiner and Alexander Wiener. This was 40 years after Landsteiner had discovered the ABO blood groups (A, B, O and AB).

The Rh blood groups are the most complex genetically of all blood type systems since they involve 45 different antigens on the surface of the red cells controlled by two closely linked genes.

The Rh system was named after rhesus monkeys which were initially used in research to make the antiserum for typing blood samples.

If the antiserum agglutinates in your red blood cells, you are Rh positive. If it doesn't, you are Rh negative.

All the ABO types can be divided into Rh positive and Rh negagtive. But people with Rh negative blood are rare compared with those who are Rh positive. In China, most Han people are Rh positive and less than 1 per cent are Rh negative.

Generally speaking, in every 1,000 Asians, only about three are Rh negative. But the percentage among people of European descent is as high as 1 in 7.

Research has shown that Rh negative blood can be given to people who are Rh positive. But it is dangerous for Rh positive blood to be given to people who are Rh negative.

"So we want more people, especially Western people, to come forward to donate blood. It would be a big help to us," Xie said.