ABO Blood Group System

The blood type we inherit is a crucial part of our genetic makeup, determining which blood components we can safely receive or donate. This categorization is based on the presence or absence of specific protein markers, or antigens, found on the surface of our red blood cells ( $\text{RBCs}$ ). The two most important classification systems are the ABO group and the Rh factor.

The ABO Blood Group System

The ABO system divides blood into four types: A, B, AB, and O.

Antigens on $\text{RBCs}$ : These are protein-oligosaccharide markers that determine the blood type.

Type A: Has A antigen only.
Type B: Has B antigen only.
Type AB: Has both A and B antigens.
Type O: Has neither A nor B antigens.

Natural Antibodies in Plasma: Your body's plasma naturally contains antibodies against the antigens not present on your own $\text{RBCs}$ . This is a key defense mechanism.

Type A: Has Anti-B antibodies.
Type B: Has Anti-A antibodies.
Type AB: Has neither Anti-A nor Anti-B antibodies.
Type O: Has both Anti-A and Anti-B antibodies.

Blood Transfusions: The Golden Rule

Transfusions require strict compatibility. The recipient's antibodies must not react with the donor's $\text{RBC}$ antigens to prevent clumping (agglutination) and destruction of the $\text{RBCs}$ .

Universal Donor (Type $\text{O}$ Negative): Type $\text{O}$ blood is considered the universal donor because its $\text{RBCs}$ lack both A and B antigens. They will not be attacked by whatever Anti-A or Anti-B antibodies the recipient may have.
Universal Recipient (Type $\text{AB}$ ): Type $\text{AB}$ individuals lack both Anti-A and Anti-B antibodies, meaning they can safely receive blood from any ABO type.

The Rh Factor

The Rh factor is a separate antigen, often called the D antigen, also found on $\text{RBCs}$ .

Rh-Positive ( $\text{Rh}^+$ ): Individuals who have the Rh antigen on their $\text{RBCs}$ .
Rh-Negative ( $\text{Rh}^-$ ): Individuals who do not have the Rh antigen.

Unlike the ABO system, $\text{Rh}^-$ individuals do not naturally possess anti-Rh antibodies.

Rh Sensitization

If an $\text{Rh}^-$ person receives $\text{Rh}^+$ blood for the first time, their body will slowly form antibodies against the foreign Rh antigen. This first exposure is rarely dangerous. However, a second exposure to $\text{Rh}^+$ blood would trigger an immediate and massive immune response, causing hemolysis (destruction of $\text{RBCs}$ ) and potential kidney damage. This is why the universal donor is precisely specified as $\text{O}$ negative ( $\text{O}^-$ ).

🚨 Common Disorders of the Blood

Errors in $\text{RBC}$ production, function, or compatibility can lead to various serious blood disorders.

1. Anemia

Anemia is a condition characterized by an insufficient number of red blood cells ( $\text{RBCs}$ ) or a low amount of hemoglobin within the $\text{RBCs}$ . This reduces the blood's ability to carry oxygen.

Iron-Deficiency Anemia: The most common form, resulting from a lack of dietary iron needed to form adequate hemoglobin.
Other Forms:

Sickle-Cell Anemia: Inherited defect causing $\text{RBCs}$ to adopt a rigid, sickle shape.
Pernicious Anemia: Caused by a deficiency in Vitamin $\text{B}12$ .
Aplastic Anemia: Failure of the bone marrow to produce blood cells.
Hemolytic Anemia: Premature destruction of $\text{RBCs}$ .

2. Rh Disease (Erythroblastosis Fetalis)

This serious condition arises from an $\text{Rh}$ incompatibility between an $\text{Rh}^-$ mother and an $\text{Rh}^+$ fetus.

First Pregnancy: During a normal pregnancy, the mother's and fetus's blood do not mix. However, some fetal $\text{Rh}^+$ blood may enter the mother's circulation during the birth process. The $\text{Rh}^-$ mother then becomes sensitized and begins producing anti-Rh antibodies.
Subsequent Pregnancy: If the next fetus is also $\text{Rh}^+$ , the maternal anti-Rh antibodies, which can cross the placenta, will enter the fetal circulation and attack ( $\text{hemolyze}$ ) the fetal $\text{RBCs}$ .
Outcome: Severely affected babies may die in utero, while less affected infants suffer from anemia and jaundice and may require specialized blood transfusions.

3. Leukemia

Leukemia refers to malignancies of blood-forming tissues, typically involving the unchecked proliferation of abnormal white blood cells ( $\text{WBCs}$ ).

Effect: The body produces vast numbers of immature $\text{WBCs}$ that are unable to perform their normal immune functions, leaving the individual highly susceptible to infection. Leukemia can be acute (rapid onset) or chronic (slow onset).