Blood products are one of the most valuable tools that doctors have to help critically ill patients. They can be lifesaving in the right circumstances, but they can also have some risks that are worth knowing about.

If you are planning to have surgery or are in a medical situation where a transfusion is being considered, understanding the basics of what blood products are, how they work, and when you should get them will help you have a discussion about transfusion with your healthcare team. In this blog, I will answer the most common questions I hear from patients about blood product transfusions.

What Are Blood Products?

Although blood looks like a simple red fluid, it’s actually an incredibly complex mixture of many different types of cells and molecules, all suspended in a water and electrolyte solution. Each of those cells and molecules have a different purpose. While it is possible to use blood straight from a donor–so-called “whole blood”–for a transfusion, we usually use just the component that is needed for a particular situation. That way, a single pint of donor blood can help many patients. The different blood products that can be made from whole blood include:

• Packed red blood cells (pRBCs): These are isolated red blood cells (also called erythrocytes). Red blood cells carry oxygen from the lungs and deliver it to the rest of the body. When patients have “low blood” or are anemic, this is the factor that they’re missing. This is also the factor most often replaced after bleeding from a surgery.
• Fresh frozen plasma (FFP): Plasma is the protein-rich fluid that cells float in to make up whole blood. With the cells removed, plasma still contains many molecules that are important, especially for clotting.
• Platelets: These are small cell fragments that are vital for clotting.
• Cryoprecipitate: This component contains specific clotting factors.

Transfusions can include one or more of these components. The pRBCs do most of the work of carrying oxygen, while the other blood products help with clotting in different ways.

What Are Blood Types?

The four blood types–A, B, AB, and O–may sound familiar. They refer to different markers present on the blood cells. There are two main markers: A and B. Some people have only one: blood types A and B. Some people have both: blood type AB. And some people have neither: blood type O. There is a second marker called Rh, which can be present (positive) or absent (negative). So each of the ABO blood types can be either positive or negative. This is all a bit confusing, I know. For a nice summary with some helpful illustrations, see this link.

Blood cells are remarkable–unlike almost any other cell in the body, they can be easily transferred from one person to another. Solid organs, skin, and other cell types aren’t as portable and require the recipient to take powerful drugs to prevent their immune systems from rejecting the donor cells. Blood does have some restrictions on who can receive it though, and the biggest restriction is the ABO blood type.

Do I Need to Know My Blood Type?

No. Before you can get a transfusion, your blood has to be tested in the hospital, and tested to look for reactions with potential donor blood. Even if you tell your healthcare provider your blood type, those tests need to be done. The exception is in emergencies, when there is no time to get test results back. In those cases, the hospital uses universal donor blood. If you want to know your blood type, though, you can ask your medical team to look up your test results.

How Is Blood Donated?

Blood is donated by volunteers who have been carefully screened for risk factors of infection. Donated blood is also tested for the most common types of blood-borne diseases, including hepatitis B and C, and HIV. If you are planning an operation in the future, it’s also possible to donate blood for yourself (called autologous donation), or for your family or friends to donate specifically for you (called directed donation). Blood products that are given by directed donation are also screened for infection. Different blood products can be stored for different lengths of time–from 5 days for platelets and to up to one year for plasma.

What Are the Risks of Blood Transfusion?

The most common risk of blood transfusion is a reaction to the blood products. Even though blood products are screened for the most common incompatibilities, it’s still possible that your body can have a reaction to other elements of the blood products. These reactions are usually mild, and include fevers and nausea. More severe reactions, such as injury to the lungs and difficulty breathing, are rare.

The risk of infection from a blood product transfusion is very low. Blood is tested for the most common blood-borne illnesses, and those tests are very accurate. Estimates are that the risk of getting a viral illness such as hepatitis or HIV from a blood transfusion is between 1 in 200,000 to 1 in 2-3 million. A blood transfusion is only given to patients who really need it, so the risk of harm from NOT getting a blood transfusion is usually much higher than the risk of any reaction to it. For more information, see this nice summary by the National Institutes of Health.

The key point is that blood is essential, but transfusions are not always necessary. The body can withstand quite a lot of bleeding without requiring a blood product transfusion, but when they are necessary, there’s no good substitute. It’s important that you talk to your team about any planned blood transfusions. That discussion should also be part of any consent for a surgical procedure. To find out more about blood products and blood transfusions, check out the excellent summaries from the Red Cross.

This story originally appeared in the  Health Dialog Care Compass Blog.