MSD Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Blood Transfusion


The Manual's Editorial Staff

Reviewed/Revised Jan 2024
Get the full details
Topic Resources

Blood carries oxygen and nutrients through your blood vessels to your organs to keep them healthy. Without enough blood, you'll feel sick and might even die.

Blood is made up of different types of cells and a liquid called plasma.

The different types of cells include:

  • Red blood cells to carry oxygen to the tissues

  • White blood cells to fight infection

  • Platelets to help your blood clot when you bleed

Plasma is mainly water. It carries minerals (electrolytes Overview of Electrolytes Electrolytes are minerals that circulate in your blood. These minerals are also in your stomach juices, in your stool (poop), in your urine, and inside your body's tissues. Salt (sodium) is... read more ) and nutrients to all the tissues in your body. It also carries useful proteins your body makes, such as those that help your blood clot or help fight off infection. Plasma also receives waste products from your tissues and delivers them to the kidneys to be eliminated in urine.

What is a blood transfusion?

A blood transfusion is when blood donated by other people is given to you through a tube in your vein.

What's in a blood transfusion?

Doctors rarely give transfusions of whole blood. Usually the lab separates out the different parts of blood so you can be transfused with only what you need. For example, you may just get:

  • Red blood cells (called "packed red blood cells")

  • Platelets

  • Plasma

But there are different blood types. If you need a transfusion, you need to get blood that matches your type.

What determines my blood type?

Red blood cells have two main chemical markers on their surface. The markers are called A and B. Your blood type is determined by which of those markers are on your red blood cells:

  • A: The A marker is present

  • B: The B marker is present

  • AB: Both the A and B markers are present

  • O: Neither the A nor B marker is present

There's also an Rh marker. If you have it, your blood type is "positive" (+). If you don't have the Rh marker, your blood type is "negative" (-). So, for example, you could be type A+ or A-.

Why does my blood type matter?

Transfusion is safest when you get blood that matches your own blood type. Getting the wrong type can be dangerous and even deadly.

Where does transfused blood come from?

A volunteer donates about a pint (450 mL) of blood at a time. The blood is sent to a blood bank for storage.

Before people donate blood, they have to answer questions to make sure their blood is safe to give to other people. They're asked about countries they've visited and behavior that could have put them at risk for certain diseases. Such diseases include hepatitis and HIV infection. After blood is donated, it's tested for certain infections. Testing makes blood transfusion very safe, although it can't eliminate all possible risk.

Sometimes, you can donate blood for yourself. For example, if you're scheduled for surgery that could involve a lot of bleeding, your doctor may have you donate your own blood a few weeks before the surgery. Then until the surgery, you'll take iron pills to help your body make more blood. If you need blood during the surgery, the doctor can transfuse your own blood.

Why would I need a blood transfusion?

You may need a blood transfusion if:

  • You've lost blood, like from an injury or during surgery

  • You have an illness or took a medicine that stops your body from making certain blood cells

You may need a plasma transfusion if you're bleeding a lot and need the clotting proteins that are in plasma.

What problems can I have from a blood transfusion?

Blood transfusions usually don't cause problems because health care providers giving the blood transfusion take precautions to keep you safe. When people do have problems, most of them are mild, but they can be serious.

The most serious, but very rare, side effects are:

  • Getting too much fluid, which can cause difficulty breathing

  • Damage to your lung that can make it hard to breathe

  • Breakdown of red blood cells because the blood type in the transfusion doesn't match yours—this can cause dark urine and a yellow color in the white part of your eye

  • Infections from viruses or bacteria in the donated blood

The most common side effects are fever and allergic reactions.


  • You may get a fever and have chills

  • Doctors will give you medicine (acetaminophen) to relieve your symptoms

  • For any future transfusions, they’ll give you acetaminophen before the transfusion starts

Allergic reactions

  • You may have itching, rash, swelling, dizziness, headache, chest pain, back pain, and fast heart beat

  • Sometimes you may also have breathing trouble such as wheezing

  • Doctors will stop the transfusion and give you an allergy medicine that relieves itching and lowers swelling

  • For any future transfusions, they may filter the donated blood to lower your chance of an allergic reaction

What safety precautions are taken during a blood transfusion?

Before your transfusion, health care providers will:

  • Test the donor's blood for certain organisms that could cause infection

  • Mix a drop of your blood with the donor's blood to make sure they work together well (cross-match)

  • Check labels on the bags of donated blood to make sure they’re the right ones

During the transfusion, health care providers will:

  • Watch you closely during the first 15 minutes of the transfusion, because that’s when you're most likely to have an allergic reaction

  • After the first 15 minutes, check on you often so they can stop the transfusion if any problems happen

quiz link

Test your knowledge

Take a Quiz!