Science has not been able to create a substitute for human blood

Science has been able to create and employ a number of medical marvels, such as the artificial heart and synthetic insulin. But not a substitute for human blood.

There have been many attempts to make artificial blood, and many people are still trying, but so far nothing has worked quite exactly like human blood."

Emmanuel Fadeyi, M.D., director of the blood bank at Wake Forest Baptist Medical Center

So, the only source of human blood remains human donors.

The need for blood is great. The American Red Cross estimates that 4.5 million people in the United States have blood transfusions annually, receiving an average of more than 40,000 pints of blood components a day. That blood is donated by approximately 6.8 million Americans each year.

But while the donation and transfusion of blood are very common, and relatively simple as far as medical procedures go, the path from donor to recipient is more complex than most people may realize.

"I think of it as a liquid transplant," Fadeyi said.

All human blood has the same components – red blood cells, which carry oxygen from the lungs to the rest of the body and return carbon dioxide to the lungs so it can be exhaled; white blood cells, which fight infection; platelets, which help blood to clot in the event of injury; and plasma, the fluid in which the other components float. But all blood is not exactly the same.

The differences lie in the presence or absence of genetic markers called antigens on the surface of red blood cells, which allow the body to identify blood cells as its own, and antibodies in the plasma, which fight cells with different antigens. The particular combination of antigens and antibodies in a person's blood is what determines their blood type.

There are 36 different blood groups, but the two most important are ABO and Rh. The ABO group includes four types – A (A antigen), B (B antigen), AB (both antigens) and O (neither antigen). The Rh has two types – positive (with Rh marker) and negative (without). Taken together, the result is eight primary blood types: A positive and negative, B positive and negative, AB positive and negative and O positive and negative. These are the types that govern what type of blood someone can receive in a transfusion.

As an example, a person with type A blood (positive or negative) cannot receive B or AB blood (positive or negative) because type A blood has anti-B antibodies that will fight the B antigens in B or AB donor blood. This interaction can cause the blood to clump, which may result in fever, more serious illness or even death.

In the vast majority of cases the donor's blood and the recipient's don't have to be exactly the same type. But they do have to be compatible. For example, everyone can receive O negative blood (universal donor type) and people with AB positive (universal recipient) can receive all eight types. But on the flip side, people with O negative can receive only O negative and people with AB positive are the only ones who can received AB positive.

Now if you don't know your blood type, you're far from alone. "Excluding people who've been in the hospital recently, I'd say less than 40 percent of people know their type," Fadeyi said.

It doesn't hurt to know your blood type, but it's not essential. If you're ever in an accident and need a transfusion, you'll receive O negative. Otherwise, your doctor will take a sample of your blood and determine your exact type and identify minor antigens (of which there are more than 350) before giving you a transfusion. And if you're going in for scheduled surgery, you can provide your own blood in advance.

It's important to note that donated blood doesn't go directly from donor to recipient. Rather, before being deemed suitable for transfusion it is typed and subjected to more than a dozen tests, primarily for the presence of viral diseases (such as HIV and hepatitis) that can be transmitted in blood.

And donated blood is rarely transfused "whole." Instead, it is separated by centrifuge into three components – red blood cells, platelets and plasma – and divided into units of 525 milliliters (roughly a pint) that can be transfused as needed into one or more patients. The white blood cells are usually removed because they can carry traces of infection, which can cause adverse reactions in recipients. Transfused red blood cells and plasma have to be compatible with the recipient's blood; platelets don't.

Red blood cells are the most commonly transfused component, primarily for trauma cases and surgery. Platelets are mostly used in cancer treatment, organ transplants and other surgeries. Plasma's primarily uses are for burn, shock and trauma patients and those with certain bleeding disorders.

The need for donated blood is constant. One reason is that blood components don't last indefinitely. Red blood cells have a shelf life of up to 42 days and must be refrigerated. Platelets are good for five days at room temperature but it takes a couple of days to test them so their effective use span is only two days, and they must be constantly but gently agitated so they don't clump. Plasma can be frozen for up to a year but must be used within five days of defrosting or thawing.

Another reason is that recipient need can far exceed donations. Though 6.8 million Americans give blood each year and "only" 4.5 million receive transfusions, most donors supply just one pint while the average red blood cell transfusion is three units, and some trauma patients can require more than 50 units of red blood cells.

Who can donate blood?

Not everybody, but the restrictions aren't too tight: You have to be at least 16 years old, weigh at least 110 pounds and be in generally good health. Also, there has to be at least 56 days between donations.

Who should donate blood?

"People with O negative blood, the universal donor type, are the ones we need the most," Fadeyi said. "And people with O and A positive, because they're the most common types.

"But we don't turn anybody away."

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