On what basis is blood classified into various groups?

An ambulance rushes its way to the E.R. with a patient in critical condition when the doctor advises that the patient needed a blood transfusion. It takes a little while before he can be transfused with blood due to some protocols.

Blood is essential to all human beings, even animals. It circulates through our body, delivers crucial substances like oxygen and nutrients to the cells, and removes metabolic wastes from the same cells.

Other substances cannot substitute blood, nor can it be manufactured.  Human donors are the only qualified source of blood for patients in need of a blood transfusion.

A person has about 5 liters of blood.  Plasma, a yellowish, watery substance, makes up about half of the blood’s content.  It contains proteins that help in the blood to clot, transport substances through the blood, and perform other functions. Blood plasma also contains glucose and other dissolved nutrients.

The other half of the blood is composed of blood cells:

  • Red blood cells, which carry oxygen to the tissues
  • White blood cells, which fight infections
  • Platelets, smaller cells that help the blood to clot

There are various tests conducted before performing the transfusion during an emergency when a patient needed a blood transfusion. One of them is to get the blood type of the patient and find a suitable donor.

How are blood classified?


Humans have varying amounts of blood, depending on the size of the person’s body; therefore, the composition of blood is not the same in each person. This is what makes a person’s blood type.

In the United States alone, over  9.5 million people are blood donors, and around 5 million patients receive blood each year.  Giving a patient the right blood type in a transfusion is very crucial because the wrong type can trigger an adverse and potentially fatal reaction. (https://www.medicalnewstoday.com/articles/218285)

A person’s blood type is dependent on the genes pass on by parents to a child.

One of the best-known ways of classification of groups of blood types is through the ABO system.  There are four major categories within the ABO group, and they are divided into eight common blood types: A, B, O, and A.B. Each group can be either RhD positive or RhD negative, which makes a total of 8 blood groups.

What makes a blood type?

Here are the four main blood groups defined by the ABO system:

  • Group A – The surface of the red blood cells contains A antigen, and the plasma has an anti-B antibody that would attack any foreign B antigen containing red blood cells.
  • Group B – The surface of the red blood cells contains B antigen, and the plasma has an anti-A antibody that would attack any foreign A antigen containing red blood cells.
  • Group O – The plasma contains both anti-A/anti-B antibodies, but the red blood cell surface does not have any A/B antigens. The absence of these A/B antigens makes it not suitable to be donated to a person with any ABO blood type. Group O blood type is the most common blood group in the world. That is why it is also known as the “universal” donor.
  • Group AB – Both A and B antigens are contained by the red blood cells, and the plasma does not have antibodies anti-A/anti-B. A person with type A.B. blood can receive any ABO blood type.

A person receiving blood from the wrong ABO group can be in a life-threatening condition. If someone coming from group B blood is given group A blood, that person’s antibodies (anti-A) will attack cells from group A. Therefore, group A blood must never be transfused to a person who has group B blood and vice versa.

On the other hand, group O red blood cells do not have any A or B antigens; it can safely be given to any other group.

The Rh Blood group system

Another blood group is the Rh blood group system. It is secondary to the ABO system in clinical importance.  The Rh antigens (C, c, E, e, D, and other antigens), especially D, are highly immunogenic.  The corresponding antibodies can result in hemolytic transfusion reactions (HTR) and hemolytic disease of the fetus and newborn (HDFN). (https://www.sciencedirect.com/topics/medicine-and-dentistry/blood-group-rhesus-system)

The red blood cells containing the RhD antigen are RhD positive, and red blood cells without RhD antigen are RhD negative.