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About 7-8% of the body weight is composed of the blood. Blood consist of different constituents such as red blood cells (RBC), white blood cells (WBC), platelets and plasma. Red blood cells or erythrocytes are the most abundant cells in the blood and make up 40-50% of the blood volume. Leukocytes or white blood cells make up only proximately 1% of the blood volume. Blood platelets or thrombocytes are tiny cells that aid in the clotting process (coagulation). Around 55% of blood volume is composed of plasma cells. Plasma is a clear fluid composed of 92% water, protein, fat, sugar, enzymes, antibodies, hormones, blood clotting factors and trace amounts of other elements.

Types of Blood Group

Antigens on the surface of red blood cells membrane can be either proteins or sugars. There are different types of antigens. Type of RBC antigen determines a blood group type. The most common blood types are ABO and rhesus types. Presence or absence of A and B antigens on the surface of red blood cells determine an individual's blood type. There are four blood groups: A, B, AB and O. Blood type O positive is the most common whilst AB negative and B negative are rare blood types. Rhesus type can be either positive or negative.

Blood Type A

A blood group is characterized by A antigen present on the surface of the red blood cells and anti-B antibodies present in the plasma.

Blood Type B

In individuals with blood type B surface of their red blood cells have A antigens while in their plasma anti-A antibodies are present.

Blood Type AB

In AB blood group both A and B antigens exist on the surface of the RBC. However, both anti-A and anti-B antibodies are absent in the plasma.

Blood Type O

Blood type O has neither A or B antigens on the surface of the red blood cells but both anti-A and anti-B antibodies exist in the plasma.

Plasma Compatibility

Different antibodies can be present in the plasma which may attack particular antigens if present. Because of that, blood group A, B or O cannot be donated to blood groups when their antibodies react against antigens of other blood groups. Therefore, individuals with A blood type cannot donate blood to an individual with B blood type and the other way around. A person with AB blood group has no antibodies against A or B antigen and can receive blood from individuals with any blood type.

Blood Type O Positive

Blood group O has antibodies against A and B antigens so people with this blood type can donate blood to people of any blood type but can receive blood only from those with blood type O. Rhesus factor will determine if blood type O is positive or negative. Type O positive is known as universal donor and can be given to recipients of A positive, B positive and AB positive blood types. On the other hand, people with O positive blood can receive blood only from O positive or O negative donors. Individuals of O negative blood type can receive blood only of donors of the same negative blood type. O positive blood type is the most common blood type. It is estimated that 38% of Americans have O positive blood type and it is mostly seen in people of Hispanic origin. There is a belief that people of blood type O positive have high stomach acid content and easily digest animal proteins, grains and dairies.

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