Blood group
Blood group is an inherited feature on the surface of the red blood cells. A series of related blood types constitutes a blood group system, such as the Rh or ABO system. The frequencies of the ABO and Rh blood types vary from population to population. In the US, the most common type is O+ (meaning O in the ABO system and positive in the Rh system), which is present in 37.4 percent of the population. The frequencies in the US (in descending order) are O+ (37.4 percent), A+ (35.7 percent), B+ (8.5 percent), O- (6.6 percent), A- (6.3 percent), AB+ (3.4 percent), B- (1.5 percent), and AB- (0.6 percent).
There are four main blood groups (types of blood) – A, B, AB and O. Your blood group is determined by the genes you inherit from your parents.
Each group can be either RhD positive or RhD negative, which means in total there are eight main blood groups.
Antibodies and antigens
Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma. Your blood group is identified by antibodies and antigens in the blood. 
Antibodies are proteins found in plasma. They’re part of your body’s natural defences. They recognise foreign substances, such as germs, and alert your immune system, which destroys them.
Antigens are protein molecules found on the surface of red blood cells.
The ABO system There are four main blood groups defined by the ABO system:
blood group A: has A antigens on the red blood cells with anti-B antibodies in the plasma.
blood group B: has B antigens with anti-A antibodies in the plasma
blood group O: has no antigens, but both anti-A and anti-B antibodies in the plasma.
blood group AB: has both A and B antigens, but no antibodies.
The Rh system
Red blood cells sometimes have another antigen, a protein known as the RhD antigen. If this is present, your blood group is RhD positive. If it’s absent, your blood group is RhD negative.
This means you can be one of eight blood groups:
- A RhD positive (A+)
- A RhD negative (A-)
- B RhD positive (B+)
- B RhD negative (B-)
- RhD positive (O+)
- RhD negative (O-)
- AB RhD positive (AB+)
- AB RhD negative (AB-)
In most cases, O RhD negative blood (O-) can safely be given to anyone. It’s often used in medical emergencies when the blood type isn’t immediately known. It’s safe for most recipients because it doesn’t have any A, B or RhD antigens on the surface of the cells, and is compatible with every other ABO and RhD blood group.
Blood transfusion
A blood transfusion is a routine medical procedure in which donated blood is provided to you through a narrow tube placed within a vein in your arm.
This potentially life-saving procedure can help replace blood lost due to surgery or injury. A blood transfusion also can help if an illness prevents your body from making blood or some of your blood’s components correctly.
blood is made up of several different parts including red and white cells, plasma, and platelets. “Whole blood” refers to blood that has all of them. In some cases, you may need to have a transfusion that uses whole blood, but it’s more likely that you’ll need a specific component.
Risks and Complications
In general, blood transfusions are considered safe, but there are risks. Sometimes complications show up immediately, others take some time.
Fever: It’s usually not considered serious if you get a fever 1 to 6 hours after your transfusion. But if you also feel nauseated or have chest pain, it could be something more serious. See your doctor right away.
Allergic reactions: It’s possible to experience an allergic reaction to the blood you receive, even if it’s the correct blood type. If this happens, you’ll likely feel itchy and develop hives. If you have an allergic reaction, it’s likely to happen during the transfusion or very shortly after.
Acute immune hemolytic reaction : This complication is rare, but is a medical emergency. It happens if your body attacks the red blood cells in the blood you’ve received. This normally takes place during or right after your transfusion, and you’ll experience symptoms like fever, chills, nausea, or pain in your chest or lower back. Your urine might also come out dark.
Delayed hemolytic reaction: This is similar to an acute immune hemolytic reaction, but it happens more gradually.
Anaphylactic reaction: This happens within minutes of starting a transfusion and may be life-threatening. You may experience swelling of the face and throat, shortness of breath, and low blood pressure.
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