A blood type or blood group is a means of classifying blood based on the presence of specific antibodies and on whether red blood cells have specific inherited antigenic substances on their surfaces. There are 38 of these blood group systems based on various antigens. These antigens can be proteins, carbohydrates, glycoproteins, or glycolipids. The most important blood group systems are those that denote ABO and Rh status, since these determine suitability of blood for transfusion. We will be discussing them in this video.
The ABO blood group system involves two antigens and two antibodies found in human blood – antigen A and antigen B, and antibody A and antibody B. The A and B antigens are found on red blood cells, while antibodies A and B are in the serum. The antibodies are usually IgM, or immunoglobulin M, antibodies. Humans all have one of four combinations of antigens, along with a corresponding combination of antibodies, which we’ll get to next. People with blood type A have antigen A, people with blood type B have antigen B, people with blood type AB have both antigen A and antigen B, and people with blood type O have neither antigen. If you’re blood type A, then you have Antibody B. If you are blood type B, you have antibody A. If you are blood type AB, you have neither antibody. Finally, if you are blood type O, you have both Antibody A and Antibody B. If antibody A encounters antigen A or antibody B encounters antigen B, an agglutination reaction results. This is very bad news and we’ll cover this in a follow up video. Blood transfusion is safe only as long as the recipient’s serum doesn’t have antibodies for the donor’s blood cell antigens.
The second most significant blood group system for blood transfusion is the Rh system. Rh stands for Rhesus and it involves 50 antigens. The most significant Rh antigen is the D antigen, as it’s the most likely to cause the immune system to react. The presence of D antigen is signified by a plus sign after the ABO blood type. Lack of the D antigen is signified by a minus sign. For example, someone who has B antigen and Rh antigen would be signified as B positive.
It’s common for someone who is D negative to not have antiD antibodies, but these people can develop these antibodies after a sensitization event. This can happen, for instance, during a blood transfusion, or during a fetomaternal transfusion of blood during pregnancy. This can result in the development of Rh disease.
So a D negative patient without sensitization to D positive red blood cells can receive a transfusion of D positive blood once. Then, they become sensitive to D antigen, so a future transfusion of D+ blood could result in a dangerous transfusion reaction. And, in the case of D negative females without prior sensitization, receiving D positive blood results in a risk of hemolytic disease of the newborn. Hence, Rh D positive blood isn’t given to D negative women of childbearing age. Meanwhile, D positive patients do not react to D negative blood. The same kind of matching is done for other Rh antigens (e.g. C, c, E, e).
Now let’s apply what we’ve learned to fill out this red blood cell compatibility table. O negative is the universal donor. Since there are no A, B, or D antigens, O negative can donate blood to anyone without triggering an immune response from A, B, or antiD antibodies. AB positive is the universal receiver, since there are no antibodies that will trigger an immune response to A, B, or D antigens.
Blood plasma is the liquid component of blood which holds blood cells and other constituents of whole blood in suspension. This is where you find A, B or antiD antibodies. Hence, plasma compatibility is the inverse of red blood cell compatibility. Someone with blood type O can receive plasma from any blood type while someone with blood type AB can only receive plasma from other people with blood type AB. Someone with blood type O can only donate plasma to others with blood type O, while those with AB blood can donate plasma to anyone.
It must be noted that, with rare exceptions, an individual has the same blood group for life. On rare occasions, it can change through addition or suppression of an antigen in infection, malignancy, autoimmune disease or due to a bone marrow transplant. If someone with a different ABO type donates bone marrow to you, your blood type will eventually convert to the donor’s type.