# Blood Group Systems This section details the major human blood group systems, focusing on the ABO and Rhesus (Rh) systems. It covers their genetic basis, the role of antigens and antibodies, principles of blood transfusion, and related clinical conditions like Erythroblastosis Foetalis. - **Blood Group Systems:** Humans have over 30 major blood group systems recognized by the International Society of Blood Transfusion. These systems are defined by the presence or absence of specific molecules, called *antigens*, on the surface of red blood cells (RBCs). - **Antigens:** Most blood group antigens are *glycoprotein* molecules. The most clinically significant systems are the ABO and Rh systems, as incompatibility here can be life-threatening. - **Antibodies (Agglutinins):** Proteins found in blood plasma that react with specific antigens. In a mismatched transfusion, the recipient's antibodies bind to the donor's antigens, causing RBCs to clump together (*agglutination*). ## 1. ABO Blood Group System Discovered by Karl Landsteiner in 1901, this system is a classic example of *multiple alleles* and *co-dominance*. ### Antigens and Phenotypes The system is based on two antigens: **A antigen** and **B antigen**. - **Type A:** Has A antigens on RBCs. - **Type B:** Has B antigens on RBCs. - **Type AB:** Has both A and B antigens on RBCs. - **Type O:** Has neither A nor B antigens on RBCs. ### Genetic Basis The ABO blood group is controlled by a single gene, **I** (*isohaemagglutinogen*), with three alleles: - ***I^A:*** Codes for A antigen. - ***I^B:*** Codes for B antigen. - ***i:*** Codes for no antigen (recessive). *Dominance Relationship:* - *I^A* and *I^B* are co-dominant, meaning both are expressed in the *I^AI^B* genotype. - *I^A* and *I^B* are both completely dominant over *i*. ### Antibodies and Transfusion Antibodies are produced in the plasma against the antigens that are *absent* on the RBCs. This production begins in early life without prior exposure. | Blood Group (Phenotype) | Antigen on RBC | Genotypes | Antibodies in Plasma | Can Receive Blood From | | :--- | :--- | :--- | :--- | :--- | | **A** | A | *I^AI^A* or *I^Ai* | Anti-B | A and O | | **B** | B | *I^BI^B* or *I^Bi* | Anti-A | B and O | | **AB** | A and B | *I^AI^B* | None | A, B, AB, O (Universal Recipient) | | **O** | None | *ii* | Anti-A and Anti-B | Only O | **Transfusion Principle:** Transfusions are based on matching the donor's antigens with the recipient's antibodies. A mismatch leads to **agglutination** (clumping of RBCs), which can block blood vessels and be fatal. ## 2. Rh (Rhesus) Blood Group System The second most important blood group system, named after its discovery in Rhesus monkeys. ### Antigen and Phenotypes - The system is primarily based on the presence or absence of the **D antigen**, also known as the **Rh factor**. - **Rh Positive (Rh^+ve):** D antigen is present on RBCs. - **Rh Negative (Rh^-ve):** D antigen is absent on RBCs. ### Genetic Basis - Controlled by the gene **D**. - **Allele D:** Codes for the D antigen (dominant). - **Allele d:** Does not code for the D antigen (recessive). | Blood Group (Phenotype) | Rh Antigen (Factor) | Genotypes | Anti-Rh Antibody | Can Receive Blood From | | :--- | :--- | :--- | :--- | :--- | | **Rh^+ve** | Present | DD or Dd | Not produced | Rh^+ve, Rh^-ve | | **Rh^-ve** | Absent | dd | Produced only if stimulated/exposed | Only Rh^-ve | ### Antibodies and Transfusion - Unlike the ABO system, an Rh^-ve person only produces **anti-Rh antibodies** after being exposed to Rh^+ve blood. This process is called *sensitization*. - **Universal Donor:** A person with **O^-ve** blood is the true universal donor because their RBCs have no A, B, or D antigens. - **Universal Recipient:** A person with **AB^+ve** blood is the true universal recipient because their plasma has no anti-A, anti-B, or anti-Rh antibodies. ## 3. Erythroblastosis Foetalis (Haemolytic Disease of the Newborn) A condition where a mother's antibodies attack her fetus's red blood cells. ### Cause and Risk Factors - Occurs due to **maternal-foetal Rh incompatibility**: - **Mother:** Rh-negative (dd) - **Father:** Rh-positive (DD or Dd) - **Fetus:** Rh-positive (Dd) - **Mechanism:** During the first pregnancy with an Rh^+ve baby, fetal blood can enter the mother's circulation (usually at birth). The mother's immune system becomes *sensitized* and produces anti-Rh antibodies. - In subsequent pregnancies with an Rh^+ve fetus, these maternal anti-Rh antibodies cross the placenta and attack the fetal RBCs. ### Complications in the Fetus - **Haemolysis:** Destruction of fetal RBCs. - **Anaemia:** The fetus becomes severely anaemic and releases immature RBCs (*erythroblasts*) into the blood. - **Jaundice:** The breakdown of RBCs produces high levels of *bilirubin*, causing yellowing of the skin/eyes and potential brain damage. - **Consequences:** Can lead to abortion, stillbirth, or a live-born baby with severe anaemia and jaundice. ### Prevention - The condition is preventable with an injection of **Rh immunoglobulin (Rhig), or RhoGAM**. - This injection contains anti-Rh antibodies that destroy any fetal Rh^+ve cells in the mother's blood before her immune system can become sensitized. - It is administered to the Rh^-ve mother during pregnancy (around 28 weeks) and within 72 hours after the delivery of an Rh^+ve baby. ## 4. Other Blood Group Systems While ABO and Rh are most critical, many other systems exist: - **H-antigen:** A precursor to A and B antigens. Individuals with the rare **Bombay phenotype (hh)** lack H-antigen and appear as Type O, but can only receive blood from other Bombay phenotype individuals. - **MNS System:** Controlled by co-dominant alleles on chromosome 4. - **Kell System:** Anti-K antibodies can cause severe haemolytic disease of the fetus and newborn (HDFN). - **Duffy System:** Antigens Fya and Fyb can be involved in transfusion reactions. - **Kidd System:** Antibodies are rare but can cause severe transfusion reactions.