Blood

Blood transfusion — the process of transferring blood or blood components from one person into another — treats massive blood loss due to trauma or replaces blood lost during surgery. It's also used to treat people suffering from conditions such as severe anemia caused by a blood disease, hemophilia, sickle-cell anemia and thalassemia.

The early history of blood transfusion was dominated by trial and error. Mostly error.

International blood factoids

• Over 80 million units of blood are donated every year around the world.
• Only 38% is collected in developing countries where 82% of the global population live.
• More than half a million women die every year from complications related to pregnancy and childbirth worldwide — 99% of them are in developing countries. Hemorrhage, which accounts for 25% of the complications, is the most common cause of maternal death.
• Up to 70% of all blood transfusions in Africa are given to children with severe anemia due to malaria, which accounts for about one in five of all childhood deaths in Africa.

So many people died after receiving the earliest recorded transfusions in the mid-17th century that the practice was banned in several European countries. Back then, doctors often used small quantities of the blood of animals, believing there was some benefit to inter-species blood exchange. Several of those early human patients survived, probably because the small quantity of blood used kept any reactions to a minimum.

Transfusion remained a risky proposition until the early 20th century, when scientists discovered that people had different blood types and mixing some types could lead to a fatal reaction. Another discovery — that refrigerated blood treated with special preservatives could be stored and used days later — made transfusion a viable treatment method.

Today, most blood transfusions do not involve whole blood but one or several of the components that make up blood. Blood is a mixture of cells and liquid, and each component performs specific tasks.

• Red blood cells carry oxygen to the body's tissues. They remove carbon dioxide. These cells comprise up to 45 per cent of your blood.
• White blood cells are the immune system's main defence against infection. They make up less than one per cent of your blood.
• Platelets are cell fragments that clot, which helps to prevent and control bleeding. Platelets make up five per cent of your blood.
• Plasma is a straw-coloured liquid that is 90 per cent water. It is vital to your survival. It provides the transportation system for blood cells. Without plasma, the cells would not be able to do their work. Besides water, plasma also contains dissolved salts and minerals like calcium, sodium, magnesium, and potassium. Plasma also carries microbe-fighting antibodies that fight disease, and makes up about 55 per cent of our blood.

The average adult carries about five litres of blood. The average unit of donated blood is half a litre.

Blood types

The eight major types of blood (and the percentage of the population that have them) are:

• O positive (about 38%)
• O negative (about 7%)
• A positive (about 34%)
• A negative (about 6%)
• B positive (about 9%)
• B negative (about 2%)
• AB positive (about 3%)
• AB negative (about 1%)

People who are O negative can donate blood to anyone. People who are AB positive can receive blood from anyone.

Blood cells are produced by bone marrow. Some cancer patients may need transfusions to build up red blood cell counts that have been reduced by chemotherapy, which can interfere with the ability of bone marrow to produce red blood cells. People suffering from hemophilia, a disease that affects their blood's ability to clot, may require plasma or the clotting factors contained in plasma to help their blood clot and prevent internal bleeding.

While the science of blood transfusion has advanced, there remain risks to the procedure. They include:

• Fever, which can be caused by a reaction between the recipient's immune system and immune cells in the donor blood.
• Allergic reactions like hives or itching sometimes happen because of a reaction between the recipient's immune system and proteins in the donated blood. These are usually mild but can be severe enough to force doctors to stop the transfusion.
• Hemolytic reaction, or the destruction of red blood cells, occurs when the donated blood and the patient's blood are not a match. This can be life threatening. It's also extremely rare as health-care professionals go to great lengths to make sure that blood types are compatible before the transfusion.

There are other risks, like the transmission of certain diseases, including HIV, hepatitis A, hepatitis B, hepatitis C and Lyme disease. (For a complete list, go to the Public Health Agency of Canada.)

Since November 1985, all blood collected in Canada has been screened for HIV and other communicable diseases. It's estimated that the risk of contracting HIV from donated blood is now 1 in 2.1 million. In the mid-1980s, that risk was 1 in 16,000. The risk of contracting hepatitis C is estimated at 1 in 1.9 million.

Canadian Blood Services and Héma-Québec have the sole responsibility for collecting blood and administering the blood supply in Canada. They have established basic criteria that blood donors must meet before they can give blood. They include that you must:

• Be between 17 and 71 years old to be a regular donor (17 to 61 to be a first-time donor).
• Weigh at least 110 pounds.
• Be in general good health and feeling well when you donate.
• Complete a screening questionnaire.

Certain people are not allowed to donate. They include:

• People who lived in Cameroon, Central African Republic, Chad, Congo, Equatorial Guinea, Gabon, Niger and Nigeria who may have been exposed to a new strain of HIV.
• People who received a blood transfusion while visiting those countries or who have had sex with someone who lived there.
• People who spent three months or more in Britain or France between 1980 and 1996. They may have been exposed to variant Creutzfeldt-Jakob disease (CJD).
• All men who have had sex with another man, even once, since 1977. Statistics show they are at greater risk for HIV/AIDS infection than other people.
• Anyone who has taken illegal drugs intravenously.

If you've recently had part of your body tattooed or pierced, you're also excluded as a blood or bone marrow donor — but for only six months.

According to the World Health Organization, at least 65 countries do not test all donated blood for HIV, hepatitis B, hepatitis C and syphilis. Tainted blood still accounts for as much as five per cent of HIV infections in Africa. The WHO estimates that six million tests that should be done for infections in donated blood are not carried out.