Blood Groups and Factors

2 April 2023

If you are a fan of medical procedurals, you are undoubtedly familiar for the call for “O Neg” for a blood transfusion.

The “O” refers to the blood type.  There are four main categories:  A, B, AB, and O.  The first three depend on the antigens, or lack thereof, on the surface of the red blood cells.  The “Neg” refers to the presence or absence of a protein called the Rh factor.  To be safe, a transfusion must match the recipient’s blood type and Rh factor.  If not, the individual will suffer a horrible reaction and may die.

About 45% of Caucasians are Type O, 51% of African-Americans are Type O, and 57% of Hispanics are Type O.  Only 7% of people are O Negative.

Whole Blood

The fluid that flows through your circulatory system is red blood cells, white blood cells, and platelets suspended in a yellow fluid called plasma.  The red blood cells carry oxygen, plasma provide clotting, and platelets are for patients who are bleeding.

Type O blood with no Rh factor is a universal donor for whole blood.  In other words, anyone can receive this blood with no adverse effects.


About 55% of blood is plasma which transports water, salts, and enzymes.  Its job is to see that all parts of the body receives nutrients, hormones, and proteins and to carry off any waste products.

For a plasma transfusion, the universal donor has Type AB blood.

Rh Factor

The Rhesus (Rh) factor is a protein on the surface of the red blood cell.  Someone with Rh positive (Rh+) can receive either Rh+ or Rh- (negative) blood within their own group safely.  A person who is Rh- can only receive Rh- blood.

The other time Rh factor is critical is when a mother is Rh- and is carrying a baby that is Rh+.  In this case, the mother’s blood could produce antibodies that would be detrimental to the baby.  Fortunately, testing for Rh is a regular part of pre-natal care and there are medical interventions that a physician can take to ensure the health of both the mother and the baby.

Clotting Factors

Coagulation, or clotting, factors are blood proteins that combine with platelets to stop bleeding.  The most common bleeding disorders are hemophilia and von Willebrand disease.  These can be genetic, inherited from one generation to another, or may develop over time.  The latter can be when something in your body, like a disease, illness, or medication, affects the blood’s ability to produce those clotting factors. As an example, a physician may prescribe a drug to prevent clotting because the patient is prone to strokes. Genetic bleeding disorder factors are designated by Roman numerals like II, VII, IX, and X.  If there is reason to be concerned, your doctor will recommend blood tests to confirm or rule out which factor is presenting the problem.  There are treatment options.

There are more than 600 identified antigens and the combination of all of these factors, plus the ABO grouping provides the details of your specific blood type.  This is part of the reason it may be difficult to find a compatible match for for organ transplants.

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