A Person's Blood Type Can Change: What Can Trigger It, and How Does It Happen?

It is possible for blood type to change, and in some cases, this can happen without a blood transfusion.

When someone is born, their blood type is determined, so everyone should know what their blood type is. Throughout our lives, there may be situations where this information is needed. Fortunately if you forget, or it is unknown, a simple test can determine your blood type. 

For most people, blood type is constant. However, in certain unusual cases, blood type can change. Of course, something specific has to occur for such an extreme change to take place in the human body. Here are some situations in which a person's blood type might change.

Medical science has long determined which blood types can be safely mixed. Some blood types are rare and difficult to replace, but fortunately, there are universal donors who can give blood to anyone. People with O-negative blood, for example, can only receive their own type but can donate to all other blood types. It's not uncommon for someone with blood type AB to receive O-type blood during a transfusion.

If enough O-type blood enters the body, nearly all of the person's red blood cells will become O-type, effectively changing their blood type. This change is temporary, ideally, the bone marrow will naturally regenerate the patient's original blood within a few months, restoring their original blood type, according to the Coffee Memorial Blood Center.

The same is true in the case of stem cell transplants. If the donor's blood type is different, the recipient's blood type may also change after the transplant. In such cases, it's common practice for the patient to receive an ID card indicating the donor's blood type as their temporary blood type. It typically takes about a year for everything to return to normal.

In rare instances, a blood type can appear to change due to illness. For example, a person with blood type A who suffers from certain bacterial infections, sepsis, colon cancer, or bowel obstruction may develop a modified substance in their blood that mimics the B antigen, triggering various reactions. In these conditions, certain bacteria in the body produce enzymes that can modify the A antigen on the surface of red blood cells. This phenomenon is known as the "acquired B" trait, and it typically resolves once the underlying condition is treated.