What are blood types?
Blood types, simply put, are the various types of human blood whose antigen characteristics determine their compatibility in blood transfusion, commonly grouped in the ABO blood group system. This article explores the classifications of blood types and what they mean.
Antigens
Antigens are foreign molecules that can potentially trigger an immune response from our body. Common antigens include proteins and large polysaccharides (carbohydrate molecules such as starch). These are often found on the surface of cancer cells, parasites, bacteria, and pollen grains.
Our immune system functions by recognizing the difference between foreign antigens and ones that the body is already familiar with. Antigens on the surfaces of pollen grains are responsible for triggering immune responses, known as allergies.
The ABO grouping system
Antigens that are on the surface of red blood cells can stimulate the production of antibodies in a person with a different blood group. Blood groups are based on the presence or absence of certain antigens.
There are many different kinds of antigen systems, but the most common is the ABO blood group, and are crucial in a medical procedure known as transfusion, where donated blood is transferred to a patient that needs it.
Inheritance
Determination of an individual’s blood type is an example of co-dominance. There are three main genotypes which will ultimately determine blood type:
- IᴬIᴬ will result in blood group A
- IᴮIᴮ will result in blood group B
- IᴬIᴮ will result in blood group AB
Since Iᴬ and Iᴮ are codominant, neither will be expressed over the other.
The final allele in blood group determination is i, where ii gives blood type O. Since i is recessive to both Iᴬ and Iᴮ, Iᴬi will result in type A blood and Iᴮi results in type B.
The blood types contain a base antigen sequence (glycoprotein) known as antigen H. In types A and B, an additional molecule is added to antigen H, differentiating it from the other two types, AB and O;
- In blood type A, this extra molecule is N acetyl-galactosamine.
- In blood type B, it is galactose.
- In blood type AB, both galactose and N acetyl-galactosamine are present.
- In blood type O, neither galactose nor N acetyl-galactosamine are present.
The recessivity of i arises due to its sole production of antigen H, so if either Iᴬ or Iᴮ is present, it will result in the addition of N acetyl-galactosamine and galactose, respectively.
Your blood carries antibodies against the antigens your red blood cells lack. For example, someone with blood type A contains type A antigens and type B antibodies, while those with blood type B have type B antigens and type A antibodies. Type AB blood does not contain any antibodies and type O contains A + B antibodies.
The Rhesus system
The Rhesus (Rh) blood type classification is a further grouping system following ABO. It depends on the Rh factor, whose presence classifies a sample of blood as either Rh positive or Rh negative. This system is highly complex, but may be generally grouped into positive and negative.
The Rh factor is associated with the ABO blood type. For example, your blood may be AB+ or B-, meaning you have blood type AB & Rh positive or blood type B & Rh negative, respectively.
Individuals with Rh positive can receive Rh positive and Rh negative blood. Individuals with Rh negative can only receive Rh negative blood. About 85% of all humans are Rh positive.
Importance
Transfusion
In blood transfusion, correctly identifying blood types is crucial. If a patient receives blood from a donor with a different blood type than their own, it can lead to an immune reaction called hemolysis, the rupturing (lysis) of red blood cells (Fig 2).
The clotting of red blood cells can block critical circulatory pathways throughout your body such as the coronary arteries or the middle cerebral artery, cutting off blood supply and potentially causing a stroke.
Childbirth
If an Rh positive man and an Rh negative woman conceive a child, there is approximately a 50% chance that the child will have Rh positive blood. If the father has Rh positive, the mother Rh negative, and the child Rh positive, there may be health issues. Normally, during the mother’s pregnancy, Rh blood type incompatibility does not usually occur, because the fetus’ blood does not enter the mother’s circulatory system.
During delivery, however, the child’s Rh positive blood may come in contact with the mother’s Rh negative blood, stimulating the mother’s blood to begin producing antibodies against Rh positive blood which may transfer to the child’s bloodstream and cause hemolytic anemia, the destruction of red blood cells leading to a dangerously low count.
In order to prevent this, the mother is given Rh immune-globulin shots during pregnancy and shortly after giving birth. These act as ‘vaccines’; preventing the mother’s body from producing Rh antibodies that can cause complications to the newborn child.
