The categorization of types is based on the presence or absence of specific antigens and the resulting occurrence of antibodies in the blood. Different systems exist, with each having its distinct blood types.
The two most extensively researched and significant systems are the AB0 system, which distinguishes between blood types A, B, AB, and O, and the Rh system, which includes the Rh+ and Rh– groups.
The ABO system
was the first to be identified, but certainly not the last. Over the following decades, numerous discoveries were made concerning the presence of antigens in blood. One of the most significant was the identification of the antigen designated with the symbol D, later termed the Rh factor.
Individuals with this antigen are Rh+, while those without it are Rh-. The Rh system operates independently of but overlaps with the ABO system.
This results in a combination of blood groups encompassing characteristics from both systems. Consequently, blood is classified as Rh+ and Rh-.
The blood group A Rh+ indicates the presence of the A and D antigens. It is the most common specification among the general population.
The inheritance pattern of the ArhD+ blood group is even more complex than that of relationships within the ABO system alone. The transmission of the Rh factor pattern must be taken into consideration.
An additional challenge arises from the existence of two variations of the gene responsible for the blood group, with the dominant trait always being Rh+ (designated with the symbol “D”), as opposed to Rh- marked with a small “d”.
The A Rh negative blood group contains A antigens but lacks identifiable D antigens. This variation is much rarer than A Rh positive.
Inheriting the A Rh negative group follows the same complex rules as previously described. It is more challenging to pass down through generations due to the dominant nature of the Rh positive factor and the recessive Rh negative factor.
In simple terms, there are eight possible combinations for a child to inherit Rh-positive blood, while only four combinations allow for the inheritance of Rh-negative blood.
Blood group B Rh plus arises due to the inheritance of specific features from parents. Still, the issue of transmitting antigens from the ABO and Rh systems should be considered separately.
This issue is extremely complicated because each antigen has two so-called alleles, i.e., variants. A mother with blood type B may have the BB allele system but also the B0 allele system. Likewise, a father with blood type B may have the BB allele system. So even if both parents have blood type B, their offspring may inherit B or O blood.
On the other hand, it is also possible that the child will have B blood, even though its parents belong to, for example, groups AB and 0. This result is only possible when 0-0 and 0-A crosses occur.
The issue of inheritance of the Rh factor is similarly complex, and in this case, the alleles are designated “D” and “d.” It is impossible to obtain Rh+ blood only if both parents are Rh- (dd).
In other combinations, which consider mixing Rh+ blood (DD or Dd) with each other or Rh- blood, a positive result is most possible.
B Rh negative blood is rare, mainly due to the dominant nature of the Rh positive and recessive Rh negative factors. In simple terms, when interbreeding occurs, the Rh-positive factor tends to replace the Rh-negative factor.
The highest chance of having B Rh negative blood is when both parents have B and Rh negative blood. However, it's not guaranteed, as the child may also end up with type O Rh-negative blood.
Type O blood is the most versatile in the basic ABO system as it can be transfused with any other blood type.
The “O” symbol signifies the absence of A or B antigens, substances present on the surface of red blood cells in many people and trigger specific immune system reactions.
Similar to other blood types, type O is inherited according to Mendel's laws. In simple terms, a child may have this blood type if:
It's important to note that each blood type (O, A, B, and AB) may also be Rh positive or negative.
The ABO blood group system is complemented by the Rh system, which is primarily defined by the presence of the “D” antigen. This antigen was identified in research on Macacus rhesus monkeys, leading to the name Rh.
Individuals with the D antigen are categorized as Rh+. Unlike the ABO system, the Rh system allows for numerous combinations, such as O Rh+, A Rh+, AB Rh-, and d.
Type O Rh+ blood is present in 31% of the population and follows the hereditary patterns explained by the ABO system. The transmission of the Rh factor involves more complex rules, including alleles denoted by “D” and “d.” Rh+ blood can be strong (DD) or weak (Dd), while Rh- is always in the dd form.
If both parents have the DD variant, the child's blood will belong to the Rh+ (DD) group. When one parent has the DD variant, and the other has the Dd variant, the child may have Rh+ blood of either DD or Dd type. If one parent has the DD variant while the other belongs to the Rh- group (dd), the possible blood types for the child include Rh+ DD or Dd. If both parents have the Dd variant, the child may have Rh+ blood (DD or Dd) or Rh- (dd). When one parent has the Dd variant and the other belongs to the Rh- group, the potential blood types for the child are Rh+ (Dd) or Rh- (dd).
The Rh-negative blood group is one of the rarest types. However, it is highly sought after for transfusions because it does not typically cause conflicts with other blood types.
A child can have a lack of the Rh factor in their blood in the following scenarios:
In the first scenario, the inheritance of Rh-blood is guaranteed. However, in the other cases, Rh- is just one of the possibilities, alongside Rh+ Dd.
In conclusion, for a child to undoubtedly inherit Rh-O blood, both parents must have both Rh-O and Rh-blood. While this is possible, it is statistically unlikely.
Rh antigens are associated with red blood cells and are not found on other cells. The D antigen is responsible for the blood type in the Rh system. People who have this antigen have the Rh+ (Rh-positive) blood group. People who lack this protein have the Rh– (Rh-negative) blood type.
The Rh system is important for pregnant women because incompatibility between the groups of mother and fetus is important in the Rh incompatibility development. This only applies when the mother has the Rh–blood group and the fetus is Rh+.
Rh incompatibility occurs when a pregnant woman's blood type is incompatible with that of her fetus. This means that the fetus has certain blood cell antigens that the mother's blood does not have. The major blood group systems involved in Rh incompatibility include Rh, AB0, and Kell, with Rh incompatibility being the most commonly discussed.
Rh incompatibility may occur when the woman has the Rh-blood group, and the fetus is Rh+. This means that the fetus's blood cells contain antigens not present in the mother's blood cells. Then, suppose a small amount of fetal blood enters the mother's bloodstream. In that case, her body may produce antibodies, which, passing through the placenta, may lead to the breakdown of fetal red blood cells (hemolysis) and its complications.
Rh incompatibility does not influence the pregnant person. In a fetus, it may induce hemolytic anemia. This condition eliminates the fetus’s red blood cells faster than it can replace them.
The effects of Rh incompatibility can range from mild to severe. These effects may also include:
The fetus may not need therapy for mild consequences. However, in severe circumstances, it may receive a blood transfusion to help replace its red blood cells.
If both parents, mother and father, have the Rh blood group, the child will also have the Rh blood group, and there will be no Rh incompatibility.
If the mother has the Rh- blood group and the father has the Rh+ blood group, the child may have the Rh+ or Rh- blood group, so there is a risk of Rh incompatibility.
If the mother has the Rh+ blood group, the Rh incompatibility will not occur regardless of the child's blood group, because the mother will not produce antibodies against the D antigen already present in her blood cells – her body will not recognize them as “foreign”.
If the mother has an Rh blood group, the doctor will schedule blood tests during pregnancy to check for antibodies (anti-D) that can lead to anemia in the baby. Additionally, the baby's blood group in the Rh system can be determined by analyzing the freely circulating baby DNA in the mother's blood.
If a pregnant woman doesn't have antibodies in her blood and the child has or may have the Rh+ blood group, she will be given a preventive drug – anti-D immunoglobulin. This procedure prevents the mother's body from producing antibodies, as the administered preparation will destroy fetal antigens that may have entered the mother's body. The drug is given as an injection in the third trimester of pregnancy (28–30 weeks).
The baby's blood group is determined after birth. If the baby has the Rh+ blood group, the mother receives another dose of anti-D immunoglobulin shortly after birth (within 72 hours). If the child has the Rh-blood group, another dose is not needed.
Women with the Rh- blood group may also need anti-D immunoglobulin administration in other situations where there is a risk of contact between fetal and maternal blood, such as vaginal bleeding during pregnancy, amniocentesis or chorionic villus sampling, miscarriage, abortion, or ectopic pregnancy. The obstetrician-gynecologist determines the decision to administer immunoglobulin and the dosage each time.
If antibodies are found in the mother's blood during pregnancy, anti-D immunoglobulin is not given. Such women need monitoring and, if necessary, appropriate treatment under the care of an obstetrician-gynecologist.
The treatment for women with the Rh blood group is similar in each pregnancy. If there is again a risk that the baby has the Rh+ blood group, the mother receives treatment with anti-D immunoglobulin unless antibodies are found in her blood.
The consequences of Rh incompatibility are borne primarily by the child. Antibodies produced by the mother can cross the placenta and destroy fetal red blood cells, which may result in hemolytic disease of the fetus and newborn, fetal hydrops, thrombocytopenia, and neutropenia. If left untreated, Rh incompatibility may even lead to intrauterine death or severe damage to the fetus.
The inheritance of blood types in the Rh system is similar to that in the ABO system. If the D antigen is present on blood cells, the blood group is Rh+; the allele determining the Rh+ blood group (D) is dominant over the allele determining the Rh– blood group (no D antigen on blood cells), which is represented by (d). Each parent passes on one allele to the child. If both parents have Rh+ blood types, or if one parent has Rh– blood type, the child can have both Rh+ and Rh– blood types. However, if both parents have the Rh–blood group, the child will also have the Rh–blood group.
It is crucial to determine the blood types of every patient expecting a blood transfusion. This involves identifying the AB0 and Rh blood types and testing for immune antibodies using standard and donor blood cells. This process ensures that the recipient does not react negatively to the donor's blood or blood component, providing a safe blood product administration.
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